The DEAD-box RNA helicase CshA is required for fatty acid homeostasis in Staphylococcus aureus

Staphylococcus aureus is an opportunistic pathogen that can grow in a wide array of conditions: on abiotic surfaces, on the skin, in the nose, in planktonic or biofilm forms and can cause many type of infections. Consequently, S. aureus must be able to adapt rapidly to these changing growth conditions, an ability largely driven at the posttranscriptional level. RNA helicases of the DEAD-box family play an important part in this process. In particular, CshA, which is part of the degradosome, is required for the rapid turnover of certain mRNAs and its deletion results in cold-sensitivity. To understand the molecular basis of this phenotype, we conducted a large genetic screen isolating 82 independent suppressors of cold growth. Full genome sequencing revealed the fatty acid synthesis pathway affected in many suppressor strains. Consistent with that result, sublethal doses of triclosan, a FASII inhibitor, can partially restore growth of a cshA mutant in the cold. Overexpression of the genes involved in branched-chain fatty acid synthesis was also able to suppress the cold-sensitivity. Using gas chromatography analysis of fatty acids, we observed an imbalance of straight and branched-chain fatty acids in the cshA mutant, compared to the wild-type. This imbalance is compensated in the suppressor strains. Thus, we reveal for the first time that the cold sensitive growth phenotype of a DEAD-box mutant can be explained, at least partially, by an improper membrane composition. The defect correlates with an accumulation of the pyruvate dehydrogenase complex mRNA, which is inefficiently degraded in absence of CshA. We propose that the resulting accumulation of acetyl-CoA fuels straight-chained fatty acid production at the expense of the branched ones. Strikingly, addition of acetate into the medium mimics the cshA deletion phenotype, resulting in cold sensitivity suppressed by the mutations found in our genetic screen or by sublethal doses of triclosan.