Résumé
In this study, growth rates and lag times of the five RNA helicase-deleted mutants of Bacillus cereus ATCC 14579 were compared to those of the wild-type strain under thermal, oxidative, and pH stresses. Deletion of cshD and cshE had no impact under any of the tested conditions. Deletion of cshA, cshB, and cshC abolished growth at 12°C, confirming previous results. In addition, we found that each RNA helicase had a role in a specific temperature range: deletion of cshA reduced growth at all the tested temperatures up to 45°C, deletion of cshB had impact below 30°C and over 37°C, and deletion of cshC led mainly to a cold-sensitive phenotype. Under oxidative conditions, deletion of cshB and cshC reduced growth rate and increased lag time, while deletion of cshA increased lag time only with H 2 O 2 and reduced growth rate at a high diamide concentration. Growth of the ⌬cshA strain was affected at a basic pH independently of the temperature, while these conditions had a limited effect on ⌬cshB and ⌬cshC strain growth. The RNA helicases CshA, CshB, and CshC could participate in a general adaptation pathway to stressful conditions, with a stronger impact at low temperature and a wider role of CshA. The DEAD-box RNA helicases are encoded by viral, ar-chaeal, eukaryotic, and prokaryotic genomes (9) and play an important role in RNA processing, transport, and degradation and in many other processes involving RNA (4, 19, 26), such as translation or ribosome biogenesis (10, 11, 22). DEAD-box RNA helicases act as molecular motors that unwind double-stranded RNA, thereby affecting the rearrangement of RNA secondary structures (9, 21). RNA helicases could also be implicated in rearrangement of ribonucleoprotein (RNP) complexes by removing protein from RNA or by the combination of both RNA-unwinding and RNA-annealing activity to promote RNA strand exchange through a potential branch migration (5, 13, 17, 24). Bacterial cells often encounter stress-ful conditions that tend to decrease the cellular fitness. Consequently, bacteria have to maintain RNA pathway functionalities and control their RNA turnover. Most of the synthesized mRNA is rapidly degraded to allow adaptation to environmental changes (14). RNA helicases could be involved in stress adaptation by maintaining and regulating RNA functions. Studies reporting the involvement of prokaryotic RNA he-licases in the adaptation to abiotic stress mainly deal with response to cold, light, and salt conditions (17). The RNA helicase CrhC maintains the photosynthetic capacity of the cyanobacterium Synechocystis. Its expression is regulated by the changes on the redox potential of the electron transport chain caused by variations in light, temperature, and salt concentrations (12). CrhC catalyzes the unwinding of RNA secondary structures but also ensures rearrangements in RNA complexes (5, 25). A Bacillus subtilis CshA homolog of Clostridium perfringens is involved in the adaptation to oxi-dative stress, with the corresponding null mutant strain showing better survival under oxidative stress conditions (2). The involvement of RNA helicases in adaptation to environmental stresses has been most often studied in response to low temperature. Thus, Escherichia coli RNA helicases CsdA and SrmB allow the correct folding of 50S ribosomal subunits at 20°C (6, 23). Bacillus cereus is a food-borne pathogen, widely spread in the environment and in a wide range of foods, which consequently has to face many physical and chemical stresses. B. cereus RNA helicases have recently been shown to be implicated in cold adaptation (3, 18). Five open reading frames were identified by an in silico analysis as encoding the putative RNA helicases CshA to CshE in the B. cereus ATCC 14579 genome. Characterization of the five deleted mutants showed that CshA, CshB, and CshC were essential for the adaptation of B. cereus at 10°C, while CshD and CshE were not (18). The aim of this work was to determine whether the Csh RNA helicase family of B. cereus could be involved in adaptation to a range of temperatures and oxidative and pH stress conditions that may be encountered by this pathogenic bacterium in the food chain.