- A new photosystem-II mutant of the green alga Scenedesmus
obliquus D₃, strain PS-28, has been shown to lack a-tocopherol
(vitamin E). The photosynthetic activity of dark grown samples of
PS-28 is about 20% of the wild-type control. Culturing the mutant
at low light intentisites (10⁴ ergs/sec-cm ²) stimulates photosynthetic
activity by as much as 3 fold. Mutant PS-28 has a high relative
fluorescence which lacks the variable yield component, but the levels
of plastoquinone A, cytochrome b-559 (H. P. ), and chlorophyll are
nearly normal. This evidence suggests that the nature of the mutation
in PS-28 is not pleiotropic, but occurs at a specific site, in the
vitamin E biosynthetic pathway.
In both mixotrophic and heterotrophic samples of the mutant
photosynthesis can be destroyed by exposure of the cells to high
intensity irradiation (10⁶ ergs/sec-cm ²). This photoinactivation is
proportional to the intensity of the irradiation, and does not occur if
treatments are performed anaerobically; thus, the damage to the
photosynthetic process occurs via a photodynamic mechanism.
α-Tocopherol, α-tocopheryl acetate or synthetic antioxidants, such
as nordihydroguaiaretic acid and N, N' -diphenyl-p-phenylenediamine,
when added to the growth medium neither stabilize the mutant against
photoinactivation nor reverse the mutation syndrome.
The capacities for hydrogen photoreduction, the production of
a 518 nm light-induced absorbancy change and PMS-mediated photophc.,
sphorylation are only moderately affected by the mutation. Also,
the above mentioned processes do not appear to be influenced by
exposure of the cells to damaging intensities of white light. Contrarily,
the rates of hydrogen photoproduction and anaerobic glucose
photoassimilation are below normal inthemutant, and these processes
show strong sensitivities to irradiation. The ferricyanide or DCPIP
Hill reactions (Photosystem-11) in contrast to the ascorbate -DC PIP
to methylviologen photoreduction (Photosystem-1) are not observed
in chloroplasts prepared from the mutant. Summarized, these
findings indicate that the mutant has a partially impaired photosystem-
II which is sensitive to high intensity irradiation treatments,
and a fully functional photosystem-I which is stable to irradiation.
The lipid and fatty acid complement in irradiated and unirradiated
samples of PS-28 were compared to similar samples of the
wild-type, and in no case was any difference noted between the mutant and the parent strain. Furthermore, several photosystem-II
mutants, possessing limited photosynthetic capacities, but having
normal levels of α -tocopherol were also found to be susceptible to
photoinhibition by high intensity irradiation treatment. These
results indicate that a -tocopherol does not function as a general
membrane antioxidant for the photosynthetic process.
The levels of vitamin E were analyzed during the greening of
mutant C -2A'. In dark grown cells of C-2.A' the level of
a-tocopherol is equivalent to that of the wild-type. After greening,
the level of a-tocopherol in the mutant is equivalent to that of mixotrophic
samples of the wild-type. Contrarily, the level of plastoquinone
A is at a minimum in dark grown cells of C-2A' and is
synthesized in parallel with the onset of photosynthesis during
greening. These observations suggest that the role of a-tocopherol
in photosynthesis is different than that of plastoquinone A, which is
a known electron transport intermediate.
A thorough consideration of the above information rules out a
role for vitamin E in photosynthetic electron transfers or phosphorylations.
The data do not support the conclusion that vitamin E functions
in the chloroplast as a general membrane antioxidant. This
suggests that toc ,plierol must either function as a site specific
antioxidant or as a structural component in or near the photosystem-
II chloroplast subunit.