Regulation of Cold Shock-Induced RNA Helicase Gene Expression in the Cyanobacterium Anabaena sp. Strain PCC 7120

Expression of the Anabaena sp. strain PCC 7120 RNA helicase gene crhC is induced by cold shock. crhC transcripts are not detectable at 30°C but accumulate at 20°C, and levels remain elevated for the duration of the cold stress. Light-derived metabolic capability, and not light per se, is required for crhC transcript accumulation. Enhanced crhC mRNA stability contributes significantly to the accumulation of crhC transcripts, with the crhC half-life increasing sixfold at 20°C. The accumulation is reversible, with the cells responding more rapidly to temperature downshifts than to upshifts, as a result of the lack of active mRNA destabilization and the continuation of crhC transcription, at least transiently, after a temperature upshift. Translational inhibitors do not induce crhC expression to cold shock levels, indicating that inhibition of translation is only one of the signals required to activate the cold shock response in Anabaena. Limited amounts of protein synthesis are required for the cold shock-induced accumulation of crhC transcripts, as normal levels of accumulation occur in the presence of tetracycline but are abolished by chloramphenicol. Regulation of crhC expression may also extend to the translational level, as CrhC protein levels do not correlate completely with the pattern of mRNA transcript accumulation. Our experiments indicate that the regulation of crhC transcript accumulation is tightly controlled by both temperature and metabolic activity at the levels of transcription, mRNA stabilization, and translation.