Influence of photoheterotrophy on the expression of chlorophyll a/b-binding proteins in the green alga Pyrobotrys stellata.

Two genes (lhca5 and lhcb1) from the unicellular, green alga Pyrobotrys (formerly Chlamydobotrys) stellata were isolated, coding for Chlorophyll a/b-binding proteins that putatively represent constituents of the light-harvesting complexes connected with Photosystem I and Photosystem II, respectively. Expression of both genes on the mRNA-level is markedly inhibited by CO2-depletion. The lhca5 transcript-level was reduced to about 25%, and the lhcb1-expression was completely blocked 9 h after removal of CO2 from the growth medium. Simultaneous addition of acetate, which can substitute for CO2 as a carbon source during photoheterotrophic growth of P. stellata, did not compensate for the diminishing effect of CO2-depletion on lhcb1. However, the amount of lhca5 transcript was comparable to that during photoautotrophic growth. These results are interpreted in terms of the specific metabolic demands of photoheterotrophic growth in P. stellata. Cyclic electron-transfer along Photosystem I must be sustained for ATP-production. Linear electron transport fed by Photosystem II and concomitant production of NADPH for CO2-reduction is no longer required.The sequences reported in this article have been deposited at the EMBL data library under the accession numbers X69434 (CSCAB1) and X71965 (CSCAB2MR).