Abstract
Chlorophyll apoprotein accumulation and expression were examined in mutants of Chlamydomonas reinhardtii blocked at specific steps of carotenoid or chlorophyll synthesis. In the absence of carotenoids: 1) apoproteins of the core and light-harvesting complexes of photosystem I (CCI and LHCI, respectively) and photosystem II (CCII and LHCII, respectively) do not accumulate; 2) mRNAs for the CCI, CCII, and LHCII apoproteins accumulate to normal levels; and 3) synthesis of the chlorophyll apoproteins is differentially affected, or in some cases, not affected. In the absence of chlorophylls: 1) the apoproteins fail to accumulate; 2) mRNA levels for CCI and CCII apoproteins are relatively unchanged; 3) levels of LHCII apoprotein mRNA, but not rates of LHCII mRNA synthesis, are reduced in a light-dependent chlorophyll-synthesis mutant (ya12); and 4) synthesis of chlorophyll apoproteins is differentially affected or not affected in the case of several chloroplast-encoded apoproteins. These results demonstrate a direct role for carotenoids as well as chlorophylls in the stabilization of certain chlorophyll apoproteins and, for others, possibly in their translation. The data also indicate a role for chlorophyll synthesis in the stability of LHCII mRNA.
Highlights
Chlorophyllapoproteinaccumulationand expression were examined in mutants of Chlamydomonas reinhardtii blocked at specific steps of carotenoid or chlorophyll synthesis
Carotenoids Are Required for Chlorophyll Apoprotein Expression/Accumulation-We established that the carotethough the pulses were of short duration (5 min), we cannot rule out the possibility, that the reduced level of isotope incorporation into LHCII apoproteins in fl is due to WT 1: very rapid degradation of the polypeptides
Transport and processing of the LHCII precursors appearedbetounaffected in the mutant as only the mature form of the protein was detected(atleast some of which hadintegratedintothe membrane), indicating thaitf increased turnover isprimarily apoproteins do not correctlyassemblewithchlorophyllsin the absence of xanthophylls, and the unassembled polypepnoid synthesis mutant fl can be used to assess the role of tides are rapidlydegraded
Summary
Chlorophyllapoproteinaccumulationand expression were examined in mutants of Chlamydomonas reinhardtii blocked at specific steps of carotenoid or chlorophyll synthesis. With the exception of an increase in anthin, which is associated with LHCII (Hagaki violaxanthin relative to theother major pigments in the dark- et al, 1986; Thornber, 1986), has been noted previously in grown cellst,he effect of illumination conditions on the overall chlorophyll b-deficient mutants
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