The carotenoids and membrane polypeptides of the light harvesting complex of mutants of Scenedesmus obliquus deficient in chlorophyll Public Deposited

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  • The photosynthetic activities, the pigmentation and the chloroplast membrane polypeptides of several mutants of the green alga, Scenedesmus obliquus, with complete or limited deficiencies in the light-harvesting complex of photosystem II are characterized and compared to those of the normal phenotype.. The principal phenotype examined in this study, K0 -9, behaves similarly to the WT Scenedesmus in that it grows autotrophically, mixotrophically and heterotrophically; cells grown under these various conditions retain near normal photosynthetic capacity without alteration of either photosystem I or photosystem II activities. Also, the associated chloroplast membrane polypeptides, as well as the polypeptides of the apoproteins of the LHC-II, appear identical to those of the WT. However, absorption spectra analyses and gel electrophoresis studies revealed that the mutant lacks the major light-harvesting complex, LHCP-II ( as shown in "green" gels) and the corresponding chlorophyll a and chlorophyll h of the light-harvesting complex. Examination of total pigment extracts of WT and K0 -9 by HPLC revealed several obvious differences. The most apparent ones noted were the absence of chlorophyll 12 and the associated decreased level of chlorophyll A, a decreased level of neoxanthin and an increased concentration of violaxanthin in extracts of K0 -9. The level of these two xanthophylls appear to show an inverse relationship with neoxanthin decreasing by approximately 70% and violaxanthin increasing by a comparable amount. This noted relationship suggests that violaxanthin, a presumed precursor to neoxanthin, undergoes only limited conversion in IC0-9. Previous mutant studies on chlorophyll 12-deficient phenotypes of algae and higher plants also noted decreased levels of neoxanthin and in vitro reconstitution studies with the apoproteins of LHC-II showed that rebinding of chlorophyll h required various xanthophylls, specifically neoxanthin, also to be present. The other light-harvesting complex mutants of Scenedesmus examined in this study, including LF-1-LHC, LF-15 hf-17-LHC, CP-13- LHC, PS-28-LHC and LF-23-LHC, exhibit similar general deficiencies of chlorophyll h, lack the green bands of the LHCP-II but retain the apoproteins of this complex and show the same relationship between neoxanthin and violaxanthin as noted for K0 -9. Even though these strains were derived from mutant phenotypes already suffering major deletions in other parts of the photosynthetic apparatus, their inabilities to synthesize or to accumulate chlorophyll J show the same common factor as noted for K0 -9, i.e., the apparent accumulation of violaxanthin and decrease of neoxanthin. This suggests that the mutual site of mutation in these various phenotypes may reside at the point of conversion of violaxanthin to neoxanthin. This interpretation supports the concept that the precursor(s) to chlorophyll b must be initially bound near to or at the site within the chloroplast membrane where it will eventually function in the LHCP. Examination by detergent polyacrylamide gel electrophoresis (PAGE) of the light-harvesting complexes of isolated chloroplast fragments of the several phenotypes employed in this investigation confirmed that the inability to synthesize chlorophyll b was paralleled by the loss of the highly pigmented LHCP-II. However, in no case was it observed that the synthesis of the apoproteins of LHC-II, or their binding to the chloroplast membrane, was affected by the mutation causing chlorophyll b deficiency. Also no changes in electrophoretic mobility, i.e., apparent molecular weight, of either the 26 or 28 kDa polypeptides were noted in electrophoretic studies on control or heat denatured chloroplast fragments of the various phenotypes. Similarly PAGE of chromatographically purified samples of the LHC apoproteins of the WT and several mutants showed no alteration of electrophoretic mobility. These observations suggest, but do not prove, that the LHC-II apoproteins of the various phenotypes studied are not modified by the mutation causing loss of chlorophyll h synthesis.
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