Light control of hliA transcription and transcript stability in the cyanobacterium Synechococcus elongatus strain PCC 7942.

The high-light-inducible proteins (HLIPs) of cyanobacteria are polypeptides involved in protecting the cells from high-intensity light (HL). The hliA gene encoding the HLIP from Synechococcus elongatus strain PCC 7942 is expressed in response to HL or low-intensity blue or UV-A light. In this study, we explore via Northern analysis details of the transcriptional regulation and transcript stability of the hliA gene under various light conditions. Transcript levels of the hliA gene increased dramatically upon a shift to HL or UV-A light to similar levels, followed by a rapid decrease in UV-A light, but not in HL, consistent with blue/UV-A light involvement in early stages of HL-mediated expression. A 3-min pulse of low-intensity UV-A light was enough to trigger hliA mRNA accumulation, indicating that a blue/UV-A photoreceptor is involved in upregulation of the gene. Low-intensity red light was found to cause a slight, transient increase in transcript levels (raising the possibility of red-light photoreceptor involvement), while light of other qualities had no apparent effect. No evidence was found for wavelength-specific attenuation of hliA transcript levels induced by HL or UV-A light. Transcript decay was slowed somewhat in darkness, and when photosynthetic electron transport was inhibited by darkness or treatment with DCMU, there appeared a smaller mRNA species that may represent a decay intermediate that accumulates when mRNA decay is slowed. Evidence suggests that upregulation of hliA by light is primarily a transcriptional response but conditions that cause ribosomes to stall on the transcript (e.g., a shift to darkness) can help stabilize hliA mRNA and affect expression levels.