Regulation of chlorophyll apoprotein expression and accumulation. Requirements for carotenoids and chlorophyll.

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.