Environmental regulation of Bacillus subtilis sigma(D)-dependent gene expression

The sigma(D) regulon of Bacillus subtilis is composed of genes encoding pro teins for flagellar synthesis, motility, and chemotaxis. Concurrent analyse s of sigma(D) protein levels and flagellin mRNA demonstrate that sig(D) exp ression and sigma(D) activity are tightly coupled during growth in both com plex and minimal media, although they exhibit different patterns of express ion. We therefore used the sigma(D)-dependent flagellin gene (hag) as a mod el gene to study the effects of different nutritional environments on sigma (D)-dependent gene expression. In complex medium, the level of expression o f a hag-lacZ fusion increased exponentially during the exponential growth p hase and peaked early in the transition state. In contrast, the level of ex pression of this reporter remained constant and high throughout growth in m inimal medium. These results suggest the existence of a nutritional signal( s) that affects sigD expression and/or sigma(D) activity. This signal(s) al lows for nutritional repression early in growth and, based on reconstitutio n studies, resides in the complex components of sporulation medium, as well as in a mixture of mono-amino acids. However, the addition of Casamino Aci ds to minimal medium results in a dose-dependent decrease in hag-lacZ expre ssion throughout growth and the postexponential growth phase. In work by ot hers, CodY has been implicated in the nutritional repression of several gen es. Analysis of a codY mutant bearing a hag-lacZ reporter revealed that fla gellin expression is released from nutritional repression in this strain, w hereas mutations in the transition state preventor genes abrB, hpr, and sin R failed to elicit a similar effect during growth in complex medium. Theref ore, the CodY protein appears to be the physiologically relevant regulator of hag nutritional repression in B. subtilis.