Regulation of CCM genes in Chlamydomonas reinhardtii during conditions of light–dark cycles in synchronous cultures

Abstract

We have investigated transcript level changes of CO₂-concentrating mechanism (CCM) genes during light–dark (12 h:12 h) cycles in synchronized Chlamydomonas reinhardtii at air-level CO₂. CCM gene transcript levels vary at various times of light–dark cycles, even at same air-level CO₂. Transcripts of inorganic carbon transporter genes (HLA3, LCI1, CCP1, CCP2 and LCIA) and mitochondrial carbonic anhydrase genes (CAH4 and CAH5) are up regulated in light, following which their levels decline in dark. Contrastingly, transcripts of chloroplast carbonic anhydrases namely CAH6, CAH3 and LCIB are up regulated in dark. CAH3 and LCIB transcript levels reached maximum by the end of dark, followed by high expression into early light period. In contrast, CAH6 transcript level stayed high in dark, followed by high level even in light. Moreover, the up regulation of transcripts in dark was undone by high CO₂, suggesting that the dark induced CCM transcripts were regulated by CO₂ even in dark when CCM is absent. Thus while the CAH3 transcript level modulations appear not to positively correlate with that of CCM, the protein regulation matched with CCM status: in spite of high transcript levels in dark, CAH3 protein reached peak level only in light and localized entirely to pyrenoid, a site functionally relevant for CCM. Moreover, in dark, CAH3 protein level not only reduced but also the protein localized as a diffused pattern in chloroplast. We propose that transcription of most CCM genes, followed by protein level changes including their intracellular localization of a subset is subject to light–dark cycles.