Plant Physiol. (1968) 43. 606-612
Studies
on
Electron-Transport Reactions of Photosynthesis
in Plastome Mutants of Oenothera
David C. Fork and Ulrich W. Heber'
l)epartment
of Plant Biology, Carnegie Institution of Washington, Stanford. California 94305
Received November 6, 1967.
A bstract. Fluoreseence characteristics and light-indueed absorbance changes of 5 plastome
of Oenothera, all having a defect in photosynthesis, were investigated to localize the
site of 'the block in their photosynthetic mechanism and 'to relate mutational changes in the
plastome to specific biochemical events in photosynthesis. In 4 of the mutants examined
photosystem 2 was largely, or completely, nonfunctional. Excitation of -system 2 did not cause
reduction of oxidized cytoohrome f in these mutants. The system-2 dependent absorbance
change at 518 mu seen in normal 'leaves was absent in the mutants. Moreover, the mutants
had a high initial fluaorescence in the presence and in the ab'sence of 3-(3,4-dichlorophenyl)-1,1dimethylurea, which did not change during illumination, indicating that the reaction centers of
system 2 were affected by the mutations. Photosystem 1 functioned normally.
A fifth mutant had an impaired photosystem 1. Even high intensity far-red light did not
lead to an aocumul-ation of oxidized cytochrome f as was seen in normal plants. Photosystem 2
was functioning, as evidenced by the fast reduction of the primary system-2 oxidant, and bN
the characteristics of the 518-m., absorbance change.
Because 1 of the 2 photosystems is functional in 'all mutants, and because they all have
the enzymes of the photossynthetic carbon cycle, it appears that the effect of the mutation
is specific. The results suggest that the plastome controls reactions within the electron-transport
chain of photosynthesis.
mutants
Genetic information has been shown to reside
onlly in the nucletus but also in other parits of
the cell. The genetic information contained in the
chloroplasts is cailled the plastome (20). A number
oif geneticallly weill-defined plastome mutants of
Oenothera were 'recently investigated (15) in an
attempt to determine the cause of photosyn'thetic
(leficiencies observed in these planits. Alll the muitants appeared to have t-he enzymes needed for the
reduction of carbon dioxide, and for the regeneration of the carbon dioxide acceptor, 'but in spite of
this they photoreduced little o,r noGCO2. The results
of the work of Halllier (14) suggested th,a,t electroni
transport might be imipaired in some way. We have
investigated electron-transport reactions in an attempt to ilocalize the site of deficiency. The results
o'btained ifrom studies on time courses of fluorescence 'as wellll -as on 1'ight-induced absorbance changes
indicated that 4 of the tmuttan,ts -have an impaired
photosystem 2, antd another appears ito have a block
near system 1. The resuilts demonstrate a relationship between the plastome anId electron transport
of photosynthesis.
Materials and Methods
not
1 On leave from the Institute of Botany. Universitv
of Dusseldorf, Germanv.
606
The photosyntheticallly-deficient plastome
mu-
tants used in this investigation were isolated from
Oenothera hookeri (plastome Ia, Ty, Th) and
Oenoth era suaveolens (plastome IIa, Thy) by WAr.
Stubbe. They are propagated by cross-pollinating
flowers from normal and imutant tissue. As the
plastid type iis transferre!d 'both by the pollen and
by the embryo sac in Oenothera, the offspring will
be heteroplastic. Somatic segregation of mutant
and normal plastids then leads to variegation. Often
variegated leaves have mutant tissue on 1 side of
the midrib and normal on the other. The development of mutant tissue requires support from normal
green parts.
For the present work the different mutant
plastid types were, combined with the genomes of
Oenothera hookeri or of the hybrid albicans X
hookeri and with the normal plastid types I and IV
(fo,r differenit plastid types cf. 23, 24, 25).
All of the mutants {are characterized by a reduced rate or complete lack of photosynthesis. Leaf
sections containing only mutant tissue are pale
green compared to normal parts. The mutants Iy
and IIy have about 50 % less total chlorophyll than
the normal, and mutants Iac, IIct, and I8, about
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6)07
FORK AND HEBER-PHOTOSYNTHESIS IN PLASTOME MUTANTS OF OENOTHERA
25 % less. The chilorophyll a to b ratios are about
the same for al!l the mutants and for normal tissuie
(14). The multant pa,rts of the plants are photosensitive and do. no-t survive prolonged exposure to
sunlight. Their biochemical and cytogenetic properties have been described (7, 14, 15).
For the experiments ithe mutant leaf tissue was
cluit away so as 'to avoid perielinal chimeras or
normal tissue. Some experiments were performed
comparing -the response of mutant tissue on 1 side
of a leaf with that of noirmal tissue on the other
side of the same lIeaf. However, since the experiments with different leaves, normall or mutant,
generally agreed very wel,l we did not often have
to use controils from other parts of the same leaf.
or even from the same volant.
For measurements of absorbance changes the
sections of mutant or normal tissue were held on
the surface of a lucite light pipe which was uised
in tihe apparatus described previously (17). Monochromatic measuring light havinfg a half-4band wid'th
of 1 m,u was obtain'ed from a Bausch and Lomb
monooh'romator having a grating wit'h 1200 lines/
mm. The actinic light was filtereld throu,gh 27 mm
o'f wq'ter andl combinaitions of colored glass and
interference-type fi,lters. Red light was obtained
by combining Cailflex C (Balzers, heat reflector)
with RG 2 (Schott). Far-red light was obtained
with Cailfilex C and RG 8, or Calf,lex C combined
with a BalIzers 'interference filter having a peak
trans-mission of 709 m,t, half-band wid,th 15 m,t.
The intensities uised were measu'red with a sil;oon
photocell calibrated with a thermopi'le.
M\ easuirements of fluorescence were made wzith
the same apparatuxs. slightly modified, so that light
which excited fluorescence was incident onI the
same side of the tleaf which faced the photomultiplier (EMI 9558B). A combination of interference
aniid colored glass filters wuith a peak transmission
of 684 nip, half-band width 15 mnu, was placed ifn
front of the photomniultiplier to transmit flutorescent
anld absoirb actinic light. Fluorescence was excitedl
by a broad band of bluie 'light which was obtained
by uising Calflex C in combination with Corning
filters 4305 and, 5562. This band had a peak
transmission nlear 440 mjL and a half band of 85 mu.
Flulorescence was also excited by a bro'ad band of
g,reen light havinlg a peak near 520 m,u, half-band
wid,th 54 m,u. The latter was obtained using Callf'lex
C in combination with Corning filters 9782, 4015
and Schott BG 18.
When needed, 'leaves were treated with 3- (3,4dichlorophenyl)-1,1-di,methylurea (DCMU) by floating sections on a 10-4 M solution for 12 hours.
All experiments were done at room temperature
(220) with air as the gas phase.
Results
Mutants Having Impaired Functioning of Photosystem 2. Reactions of the f-Type Cytochrome.
0
403
353
-20
420
-3
550
500
450
400
Wavelengfh,mp
FIG. 1. Light-mintus-dark difference spectra for
leaves of muitant 1T of Oenothera. TPhe spectrum with
circles gives the response obtained after 3 seconds of
illumination and the spectrum witih triangles after 0.1
second of illumination with light of 709 mju (14.2 neinstein cm-2 sec -1)
To test for system-2 activity in the mutants we
examined tthe -reactions of ithe f-type cyt'ochrome
since it has been well d-ocumented (2, 3, 9, 13, 19)
that this compound fuinctions as a red-ox carrier in
the electron-transport chain between the 2 photochemicail systems. It iis oxidized by system 1 and
reduced by system 2. The light-minuis-dark difference spectrtum for mutant IT (fig 1) is characteristic of that produced ulfpon excitation of an f-type
cytoc,hrome and had a large nega'tive Soret band
slightly below 420 mt, 'a pos,iti-ve band near 403 mn
Normal
~~~~a
s
a
Mutont I &
709 on
E 709on
0
4~~~~~
N
Off
651 on Off
0
I
u
,&I/, = +5xl0-3
_
i
-a
-0
c
-0
a
d,
709 on
4
709on
651ion
t
;+f
-c
b
651 on
_orm ;
t
$
709off 651 off
651 off
a
*
709off
0
5
10
15
20
Off
0
5
10
e
15
20
Time,sec
FIG. 2. A comparison of the kinetics of light-induced
absorbance changes at 420 m,u caused by oxidation of
the f-type cytochrome in leaves of normal Ocuiothcr-a
and mutant Ij. The wavelength of the actinic light
was 709 m,c (14.2 neinstein cMu2 sec-1 for traces a and
b and 7.2 neinstein cm-2 sec-1 for traces c, d. and e).
The intensity of the 6l- m,u light was 2.8 neinstein
cm-2 sec1.
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608
68PLANT
and ain
a band at 552
positive 518-m,u bands
mpl.
PhIYSIOLOGY
The negative 475 and
in the difference spectrutm for the initial deflection produced tupon illumination are discuissed in another section of the
resull ts. It is clear that the
bandl of cvtochronme
f is suiperimposed on other large positive changes
in thie green region which
w1ill not be discussed
here. The shoulder aroundi(l 435 m/ may be cauisedl
ly oxidation of P700 (or of a b-type cytochrome).
'The alntagonistic effects of far-red and red light
oni the oxidation state of cytochrome f (meastured
at 420 m,u) in a normal leaf are shown in figuire 2.
Far-red light (709 m/) cauise( rap.d loxidationi of
the cytoc,hrome (seen in trace
a downwardl
(leflection). Dark reductioni of the cYtocbhrome
after 709-m/ light was sloNv. Trace b of figuire
sho.w,s the effect of tuirning off 709-mM lighit but at
the same time turning Onl 651-mtu lighit. In this
case rapid reduction of cytochrome was seen since
651-mM light excited systemii 2 effectively. The
intensity of 651 m/t was low enoulgh so that this
beam alone did not prodluce an appreciable deflectioni
(cf. trace b la.bellled "651 off"). The accelerating
effect of switching 651 for 709-mu light upoIn t'he
reduiction of tihe cytocfhrome in normal leaves was
abolishedl after inicuibatioIn in DCMU. AMoreover,
after this treatment 651-mnu light alone produced
slow oxidation of the cytochrome similar to that
seen
a
a
as
No DCMU
f
+Of
+DCMU
Normal
NOff
E
*
On
a1a
bb
*
On
T
AI/, =10-2
Mufonf Hi'
,Off
-o
Off
0
0
-o
a
On
c
Off
I
AI/x10-3
Mufant ll
*
'Xf"
Off
On
0
d
On
5
~~~~~~~4
On
4
e
10
0
f
5
10
Time,sec
FIG. 3. A comparison of the kinetics of the lightilduce(d ahblsorhance changes at 548 mn, in leaves of
niormcal Ocnothera anid mutanits ITy and hIa. Actinic
illumination was provided 1w- broald band (620-80 mi)
of red light of about 2.6 X 10' ergs cm l-2 eeC .
a
prodluced by 709-mM light.
The accelerating effect of 651-mn light oni the
reduction of cytochrome f coultld not be lenlonstrated for muitant To. Figure 2 (trace c) i'l;luistrates that in mutant 1I, as in ithe normal lIlant,
illutminatioon with 709-mtt light l)roduced a fast oxidationi wvhich was followed by slow re(duiction inI
the (lark. Illuimination of multant Th wvith weak
651-m,u light alonie p>rolhced a sloxv oxidlation
(trace d) and wheni 651-im/.L
switched fuor
709-m/, light (trace c) there was no reduiction an(l
the cytochriome remained oxidized. System 2 thuis
appears not to fulnictioni in this muttanit. The sanme
general behcavior was foulind for multtanlts Ia, Iy,
and Ily indicating that in these niiitanits photosystenm
2
is
also essentially nonfunctional. Difference
spectra oitainled by illuminiation of these muiitaints
witlh re(l o,r far-red light were almost identical aId(
exhibited the same chlaracterist.cs as those given
in figulre 1 for multant lb.
Miitant ly had, in
addition, a smalil positive peak at 430 mM suggesting
reducetion of a cytochrome of the b type.
Absorbanec Chan'lcs (it 58,ip.. Absolrbance
chaniges seen at 518 ma, in normiial Ocnothcera leaves
fig 3) are complex butt typical of tho.se
(trace
produtce(l by other green planits containing chlorophyll b (11, 12, 27). Illtumination with a broa(l
band of red lig,ht whic,h excited both system 1 aniid
2 produiced, initially, a fast rise which was followed
by a Ilowver, and larger, inicrease to a steady state.
Leaves poisoned witih DCAMU still ha(l the fast, but
smaller, initial increase andl the steadv-state cihange
Nvas ailniost completely inhibhited (trace b).
\ctioiu
a
was
a
spectra donie (11)
on
the
green
alga Ulva lobata
have shown tha,t the fast, iniitiall increase with
or withou1t DCMU as well as the low steady-state
change persisting in DCTMU are sensitized by system 1. The slow, larger absorhance inkcrease is
sensitized by system 2.
Mutant Ily had a 518-m,m change like that of
normal plants whose system-2 activity had been
blocked by treatment wi,th DCMU. This plant had
identical 518-m,u absorbance changes in the presence
or absence o.f DCMTU (traces c and d) indicating
that system 1 alone is functional in this plant.
Very similar responses at 518 mp, were found for
sonie other muititants (Iy, 1h and la). The relater
spouses of mulftant Ila are discussedI in
sectiOn1
The observations Onl the behavior of cytochrome
f ini mutant
(and also mutants y, ITa, and Hly)
as welil as those oIn ithe 5l8-mp. change discousssed
at,bove aill suggest that svstem 2 is almost completely
nonfuinctional in these plants. Ftirther evidence
tfhat system 2 was not functioning was ob)tained
from their fluorescence behavior.
Timte Courses of Fluorescence in7 DCMU-Treated
Leaves. There is evidence (4, 5, 6, 8, 10) that the
fluiorescence o,f chilorophyll in algae ancd higher
plants originates mainly fro,m system 2 and th,at tlhe
fluorescence increase which occurs around 685 mn.
duiring illumination reflects the oxidation state of
the primar) pho,tooxidant (Q) of system
(10).
When Q is in the oxidized form uit quenches chlolrophy ll flulo,rescence and whein in bhe reduced state
a
.
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609
FORK AND HEBER-PHOTOSYNTHESIS IN PLASTOME MUTANTS OF OENOTHERA
Mutant f l(No DCMU)
100
Mutant Ul.(No DCMU)
75Normal (DCMU)
0)
° 50
-
0)
25 -
0
5
0
10
IS
20
25
30
Dark fime,sec
FIG. 4. Fluorescence at 6&4 mg produced immnediately upon illumination of niormal leav-es of Qenothera
and of mutanits HJy and 11a a-s a function of dark interval l)etween exposures to blue actinic light (267 ergs
cm-2
s-ec-i).
(QH) it l)roduices increased fluorescence. In the
albsence of DCMU, oxidation of QH may be bro-ught
aibout by excitation of system 1. QH may ailso be
oxidized by a dark reaction that proceeds in the
presence or absence of DCMU.
In the normal leaf treated with D-CMU the
fluorescence *at 684 mJL produced initially was low.
Duiring ililumination the fluorescence rose rapidly
to a higher level as Q was reduced by system 2.
Figure 4 showrs that in DCMU-treated normai
leaves QH was half reoxidizeda in the dark in about
0.7 sec. A number of mutants ('Ia, Iy, Ib, and
(Hy) were found to have a high initial fluorescence
at 684 m/ comlpared
r
to normal plants whether
poiso-ned with DCMU or not. Moreover, this highi
fluorescence seen initially did not increase further
during illumination and was not decreased even by
a long dark interval (10 mm). Figure 4 shows
this wacak
of dark reoxidation of QH for mutant
II)Y.A
DCMU indicating that reduction of Q is at leaslt as
effective in this mutant as in normall leaves. However, initiail f(luorescence after a 'long dark period
was higher in tihe mutant than in the normal leaves
and t'he mutant had somewhat less varia)ble ftluorescence. Therefore, the number of system-2 reaction
centers may be somewhait reduced in mutant Iha
when compared with the normal.
AbsorUance Change at 5i8 m,u. Further evidence for the functioning of system 2 in mutant
IIa wa,s seen in the behavior of the 5l18-mMu absorrbance change. Trace e of figure 3 shows that the
change before poisoning with D'CMU consisted of
a fast, initial rise fol,lowed by a second, slower and
somewhat 'larger increase, characteri,stic of photosystem 2 acitivity. Thi,s second phase was smaller
than in the normal plant. Trace f shows the effect
of incubating the leaves in DCMU. A's in the
normall plant the fast, initial transient was not
inhibited, but even increased, and the second rise
wa's abolished.
Cytochromiie f. The 'beihavior of the f-type
cytochrome suggested that mutant hIa had a block
in electron transport near system 1. In thli,s mutant
we were tunable to see accumulation of oxidized
cytochrome f with 709-m,u actinic light as in the
normal and mutant lb (cf. fig 2). In mutant HaI
even ia broad band of 'high intensity far-red light
gave no response until the leaves were incubated
in DCMU. After this treatmenit accumulation of
oxidized cytoch'rome f occurred both in high and
low iintensity far-red ilight and the difference spectrum had negative bands at 553, 420 and a positive
band at 405 mr,. Figure 5 compares the rate of
cytocthrome f oxidaltion as a futnction of intensity
02
Muttant With Imipaired Functioning of Sys-
tem i. FlAuorescence. Mutant 1ha was found to
have a fluorescence behavigorintermediate between
mautant ly or I>l and the normal plant. Figure 4
shows that for mutantIda QH was ha,lf reoxidized
in the dark in a(bout 0.6 sec. The fluorescence
behavior o-f this mutant was siimilar wi-th and
without DCMU. Thus, unlike the other mutants
described so far, mutant Ilae retains funrcFtional
systaem-2 reaction centers because after Q is formed
in a dark reaction it may be reduced again in the
li.ght.
To es-timate how welil photosystem 2 was functioning the half time of the increase oif variable
filuorescence, whidh reflects the reduction of Q,
was meassured at different light intensities. The
rise, as judged from the half-time of QH formation,
was found to be 30 to 50 % faster in DCMUAtreated
mutant Tla than in normal -leaves treated with
C\
o O
_W
2
4
6
8
10K104
Intensity,ergs cm 2 sec'
FIG. 5. The rate of decrease of absorbance at 420
for differences in the chlorophyll
content of the leaves) caused by oxidation of the f-type
cytochrome in normal leaves of Oeitothera and mutants
IIhy and 11a as a function of the intenisity of a broad
band of far-red light (675-800 m,u). The leaves were
treated with DCMU as described in the text.
mA (unoorrected
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610
PLANT
PHYSIOLOGY
of far-red light for normal Oeuothera, mutant Ily
ancd Ila after treatment with DCMU. It is clear
from this figure that the efficiency of cytochrome
oxidation in mutant IIa is very low in comparison
wvith normal OQnothera or muttant IIly. The differences apparent in figure 5 between mtttant hIa
onl the 1 hand and normal Oenothera, or mutant Ihy,
on the ot-her were even mulch more pronounced if
electroIn flow from photosystem 2 was not blocked
by DC'MU. Then excitation of system 2 by far-red
light (5) was sufficient to couinterac.t oxidation of
cytochrome f in multaint Tla buit not in other plants
tion was as efficient in the mutant as in normal
leaves indicating that system 1 is functioning normally. Experiments with isolated chlloroplasts from
the mutants also demionstrated that system 1 is
functioning normally since high rates of ATP
formation in a cyclic type of photophosphorylation
were observed 'using PMS as a cofactor (14).
Apart from a lower pginent content and increased
fluorescence intensity, low-temperatture absorptionl
spectra -and filuorescence-emission spectra of the
mutants (not shown here) failed to revea.l abnormalities when com,pared with spectra of normal
having normal system-i
leavets.
activity.
The quanituim requ1irements for cytochrome oxi(lation were 10 for mutant ily, 16 for multant ly,
30 for normal Ocnot hera, and 170 for muitanit TIa.
Stuch measurements in highly-scattering leaves are
sullbject to relativelly large errors especially in the
strongly-absorbing 420-mu region (1), but their
relation shows again impaired functioning of photosy-stem
in muitant ITa.
Discussion
Mutants Hazing Impaired Photosysteml-2 Activity. The experiment's described in the first section of the Results demonstrate that mutants la,
Ty, 1h and ITy have a 1block in photosystem 2 which
is responsible for their inabiliity to perform photosy nthesis at a significant rate. The rate of l-ight(lependenit 14CO. fixation by these mlu'tants was less
than 1 % of t'he rate of normail ;leaves (15) except
for mutant Ia which, like multant Ila, couild almost
compensate respiration (Hallier, personall commtnication).
The initiall flu1rescesnce of unt,reated
poisoned multants (as txypified by
mutant
or
DCMIU-
Ily, fig 4)
high and did not significantly increase d.uring
illumination. Dark 'intervailis or illumination with
far-red light did not lower this high initial fluorescence. This was even true for mutant Ia. As
the ilight-induced rise in filuorescence reflects the
trapping of energy by 'the reaction centers of system
2 (10), these ceniters welre all (multant ly) or
nearly all (mutant 1a) nonfuinctional (reduiced and
inecapable of reactiiig back in the (lark) or perhaps
absent altogether.
The inability of the photooxidant of system 2
to function as ain efficient trap for energy absorbed
by the 'light-harvesting pigment molectules was also
seen in the behav,ior of the 518-m,u absorbance
chan'ge. That part of the change produced upon
was
excitation
of
systemn
2
in
normal
leaves
(11)
was
absent in the 'mutants. In addition, cytochrome f
oxi(lized bv far-red lighit was not, as in the normal
plant, reduced UpoIn excitation of system 2. Thuis,
in these respects, the uinlpoisoned mutants behaved
just 1'ike normal plants treated with DCTMU.
It appears that the effect of the mutation was
specific for photosystem 2. Cytochrome f oxida-
The enzymes of the photosynthetic carbon cycle
are present in the mutants and have abouit normal
aotivities. The very slow formation of radioactive
sugtar phosphates from 14CO2 by the mutants (15)
can be accounted for, at 'least in part, by photosystem-1 activity. The ATP content of mutant
tissue increased tupon illumination (14), apparenitly
due to svstem-i mediated photophosphorylation.
Photooxidation of ascorbate by system 1, or perhaps
transport of reducing equivalents from the cytoplasm, may lead to the slow formation of some
NADPH.
The reduiced ch'lorophyll. content of the leaves
may be considered as a secondary consequence of
the mutation. A block in the electron transport
chain, which prevents 'light energy from being
effectiveily channeled into photosynthesi!s, may be
expected to cause photooxida'tive pigment destruction as observed in the mutants (14). Likewise,
starvation phenomena stuch as increased .appearanice
of free amino acids may also be considered as
secondary events (15).
A Mutant Hazving Imp/'aired Photosvstent-r Actizvitv. Mutant ITa wN-as different from all]l the
other mutants investigated in that it had impaire(l
functioning of system 1. In this multant even a
high intensity far-recl light was insufficient to cause
appreciahle oxidation of cyto'ch'rome f. A pronouinced oxidatilon coulld only be observed 'after
addition of DCMU. Apparentliy there was sufficient excitation of photosystem 2 even in far-red
light to couniteract the accuimuflation of cvtochrome
f by system 1. In mu1tant Ha 'the rate of cvtochrome oxidati'on in far-red light after bilocking
electron flow from sysitem 2 wvith DCM\U was about
12 times 'less onI a uinit chlorophyll basis than in the
normal or in mutant IIy.
The 518-mpt change of muitant Ila liad the fast,
transient system-1 component as wel;l as a distinct
second, slow phase, sensitive to DC\IIU, characteristic of tihat produced tupon excitation of sxstem 2.
Photosystem 2, therefore, is operative in thfis miltant.
However, it does not appear to be comipletely uinchanged in its activity. While the half times of
the rise in fluorescence in DCMU-treaited leaves
indicate even faster redtuction of QH in mutant
IHa than in normall leaves, the nuimber of 'trappina
centers of system 2 appears to be .somewhat redtuced,
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FORK AND HEBER-PHOTOSYNTHESIS IN PLASTOMIE MUTANTS OF OENOTHERA
perhaps as a secondary consequence of the mutation. As discussed above, it is cjlear ithat photooxidative reactions induced by a block in system I
should secondarily influence photosystem II. StilIl,
the activity of photosystem 2 is much higher th-an
that of photosystem 1 and the liatter is clearly
limiting photosynthesis. Mutant IIa has an appreciable residual photosynthetic activity and can
almost compensate respiration. As wiith the ot'her
mutants, it conntains all the enzymes of the photosynithetic carbon cycle in about normal activities.
Genetic-Physiologicail Aspects. There is an
overwheilming body of evidence to ilink point changes
of the genome caused by mutations to specific
alterations of individual proteins. One of the aims
of the present investigation was to determine
whether it was possible to link, in a similar way,
the changes caused by mutation of the plastome to
specific events in the biochemical machinery of
chloroplasts. Spontaneously-occurring plastome mutants are particularly well suited to prove tha-t the
m-utated genetic material is associated closely with
chloroplasts. The main criteria (cf. 18) are quantitative differences in the extent of variegation in
hybrids from reciprocal crosses between normal and
defective types. The nature of the plastome is still
unknown. DNA Ihas 'been 'identified in recent years
a constituent of chloroplasts and evidence for a
relationship between chloroplast DNA and chloroplast development has been presented (21). Fturthermore, chloroplast DNA appears to be engaged
in the formation of chloropl-ast ribosomes (22).
However, in all mutants deficient in photosynthesis,
listed in a recent survey by Kirk (16), that have
been investigated biochemically and genetically, the
muitations appear to be centered in the nucleus. The
results of the present work indicate that the plastome exercises control over specific chloroplast
reactions. Since none of the photosyntheticallydeficient mutanits investigated had a 1block in its
carbon metabolism, and all were affected in their
electron-transport chain, it appears that the plastome control steps of electron transport which occur
in the lamellar struictture of chloroplasts.
as
Acknowledgment
It is a pleasure to thank Prof. Dr. XV. Stubbe who
kiindly supplied the mutants used in this work and who
provided valutable criticism of certain parts of this
manuiscript.
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