Environmental regulation of Bacillus subtilis σ(d)-dependent gene expression

The σ(D) regulon of Bacillus subtilis is composed of genes encoding proteins for flagellar synthesis, motility, and chemotaxis. Concurrent analyses of σ(D) protein levels and flagellin mRNA demonstrate that sigD expression and σ(D) activity are tightly coupled during growth in both complex and minimal media, although they exhibit different patterns of expression. We therefore used the σ(D)-dependent flagellin gene (hag) as a model gene to study the effects of different nutritional environments on σ(D)-dependent gene expression. In complex medium, the level of expression of a hag-lacZ fusion increased exponentially during the exponential growth phase and peaked early in the transition state. In contrast, the level of expression of this reporter remained constant and high throughout growth in minimal medium. These results suggest the existence of a nutritional signal(s) that affects sigD expression and/or σ(D) activity. This signal(s) allows for nutritional repression early in growth and, based on reconstitution studies, resides in the complex components of sporulation medium, as well as in a mixture of mono-amino acids. However, the addition of Casamino Acids to minimal medium results in a dose-dependent decrease in hag- lacZ expression throughout growth and the postexponential growth phase. In work by others, CodY has been implicated in the nutritional repression of several genes. Analysis of a codY mutant bearing a hag-lacZ reporter revealed that flagellin expression is released from nutritional repression in this strain, whereas mutations in the transition state preventor genes abrB, hpr, and sinR failed to elicit a similar effect during growth in complex medium. Therefore, the CodY protein appears to be the physiologically relevant regulator of hag nutritional repression in B. subtilis.