Osmotic Stress Induced by Carbohydrates Enhances Expression of Foreign Proteins in Escherichia coli

Arabinose has been serendipitously observed to enhance the expression of P450s in Escherichia coli. To understand the mechanism of this arabinose-dependent enhancement, the effects of various carbohydrates were investigated. Surprisingly, a series of sugars, including pentoses and hexoses, enhanced the foreign gene expression in a manner similar to arabinose. Furthermore, glycerol, a poor carbon source, also enhanced P450 expression. These results indicate that the enhancement is independent of the specific efficiency of the carbon source and also suggest the involvement of osmotic stress. Therefore, the effect of the ςs (also termed ς38) factor, a sigma subunit of RNA polymerase that plays a central role in regulating the expression of osmotic stress response genes, has been examined. We found that the glycerol-dependent increase in P450 expression was not observed in ςs-deficient E. coli, indicating that carbohydrates enhance the foreign gene expression in E. coli via the induction of the osmotic stress response. The results suggest the important role of the osmotic stress response in posttranscriptional processes required for producing functional proteins.