A cold shock-induced cyanobacterial RNA helicase.

The ability to modify RNA secondary structure is crucial for numerous cellular processes. We have characterized two RNA helicase genes, crhB and crhC, which are differentially expressed in the cyanobacterium Anabaena sp. strain PCC 7120. crhC transcription is limited specifically to cold shock conditions while crhB is expressed under a variety of conditions, including enhanced expression in the cold. This implies that both RNA helicases are involved in the cold acclimation process in cyanobacteria; however, they presumably perform different roles in this adaptation. Although both CrhB and CrhC belong to the DEAD box subfamily of RNA helicases, CrhC encodes a novel RNA helicase, as the highly conserved SAT motif is modified to FAT. This alteration may affect CrhC function and its association with specific RNA targets and/or accessory proteins, interactions required for cold acclimation. Primer extension and analysis of the 5' untranslated region of crhC revealed the transcriptional start site, as well as a number of putative cold shock-responsive elements. The potential role(s) performed by RNA helicases in the acclimation of cyanobacteria to cold shock is discussed.