Irradiance-Dependent Changes in the Size and Composition of the Chlorophyll a-b Light-Harvesting Complex in the Green Alga Dunaliella salina

Irradiance-dependent adjustments in the size and composition of the Chi a-b light-harvesting protein complex (LHC-II) were investigated in the green alga Dunaliella salina. Cells grown under low irradiance (100 μmol photons m−2s−1) had a low Chl a/Chl b ratio, and an abundance of cellular Chl and LHC-II apoproteins compared to cells grown under high light intensity (2,200 μmol photons m−2s−1). In low-light grown cells, four apoproteins of the LHC-II migrating to 32, 31, 30 and 28.5 kDa were termed according to their apparent molecular mass as LHC-II-1, LHC-II-2, LHC-II-3 and LHC-II-4, respectively. In thylakoid membranes from high-light grown cells, LHC-II-1 was practically missing and LHC-II-2 was greatly depleted. On a cell basis, the steady-state amount of total LHC-II apoprotein in high-light grown cells, measured either from the intensity of protein staining by Coomassie brilliant blue or from Western blotting, was 5–7% of that in low-light. In vivo rates of apoprotein biosynthesis were measured from the time course of [35S]sulfate incorporation into LHC-II. On a cell basis, the rate of biosynthesis in highlight grown cells was about 20% of that in low-light grown cells. Thus, in high-light grown cells, the rate of LHC-II apoprotein biosynthesis was ˜3 times greater than the steady-state amounts of LHC-II present in thylakoids. The results suggest discrete irradiance-dependent transcriptional and posttranslational regulation steps that define the amount and composition of the LHC-II in chloroplasts.