Cold-induced putative DEAD box RNA helicases CshA and CshB are essential for cold adaptation and interact with cold shock protein B in Bacillus subtilis

The nucleic acid binding cold shock proteins (CSPs) and the cold-induced DEAD box RNA helicases have been proposed separately to act as RNA chaperones, but no experimental evidence has been reported on a direct cooperation. To investigate the possible interaction of the putative RNA helicases CshA and CshB and the CSPs from Bacillus subtilis during cold shock, we performed genetic as well as fluorescence resonance energy transfer (FRET) experiments. Both cshA and cshB genes could be deleted only in the presence of a cshB copy in trans, showing that the presence of one csh gene is essential for viability. The combined gene deletion of cshB and cspD resulted in a cold-sensitive phenotype that was not observed for either helicase or csp single mutants. In addition to the colocalization of the putative helicases CshA and CshB with CspB and the ribosomes in areas surrounding the nucleoid, we detected a strong FRET interaction in vivo between CshB and CspB that depended on active transcription. In contrast, a FRET interaction was not observed for CshB and the ribosomal protein L1. Therefore, we propose a model in which the putative cold-induced helicases and the CSPs work in conjunction to rescue misfolded mRNA molecules and maintain proper initiation of translation at low temperatures in B. subtilis.