Light-harvesting complex gene expression is controlled by both transcriptional and post-transcriptional mechanisms during photoacclimation in Chlamydomonas reinhardtii

To compensate for increases in photon flux density (PFD), photosynthetic organisms possess mechanisms for reversibly modulating their photosynthetic apparatus to minimize photodamage. The photoacclimation response in Chlamydomonas reinhardtii was assessed following a 10-fold increase in PFD over 24 h. In addition to a 50% reduction in the amount of chlorophyll and light-harvesting complexes (LHC) per cell, the expression of genes encoding polypeptides of the light-harvesting antenna were also affected. The abundance of Lhcb (a LHCII gene), Lhcb4 (a CP29-like gene), and Lhca (a LHCI gene) transcripts were reduced by 65 to 80%, within 1-2 h; however, the RNA levels of all three genes recovered to their low-light (LL) concentrations within 6-8 h. To determine the role of transcript turnover in this transient decline in abundance, the stability of all transcripts was measured. Although there was no change in the Lhcb or Lhca transcript turnover time, the Lhcb4 mRNA stability decreased 2.5-fold immediately following high-light (HL) stress. The Lhcb transcript abundance, on the other hand, was primarily dictated by the rate of transcription as determined using an arylsulphatase reporter gene system. Transcription from the Lhcb promoter was initially repressed in HL but recovered to the LL rate after 6-9 h. Interestingly, the LHCII and CP29 transcripts recovered to their prestress levels before there were significant reductions in the abundance of their corresponding polypeptides. Although there are short-term alterations in transcription and transcript stability, the long-term acclimation of the light-harvesting antennae to HL occurs primarily at the translational level.