sigma B of the gram-positive bacterium Bacillus subtilis is an alternative transcription factor activated by a variety of environmental stresses, including the stress imposed upon entry into the stationary growth phase. Previous reports have shown that this stationary-phase activation is enhanced when cells are grown in rich medium containing glucose and glutamine. The sigma B structural gene, sigB, lies in an operon with three other genes whose products have been shown to control sigma B activity in response to environmental stress. However, none of these is sufficient to explain the enhanced stationary-phase activation of sigma B in response to glucose. We show here that the four genes previously identified in the sigB operon constitute the downstream half of an eight-gene operon. The complete sigB operon is preceded by a sigma A-like promoter (PA) and has the order PA-orfR-orfS-orfT-orfU-PB-rsbV-rsbW-sig B-rsbX, where rsb stands for regulator of sigma-B and the previously identified sigma B-dependent promoter (PB) is an internal promoter preceding the downstream four-gene cluster. Although the genes downstream of PB were also transcribed by polymerase activity originating at PA, this transcription into the downstream cluster was not essential for normal induction of a sigma B-dependent ctc-lacZ fusion. However, deletion of all four upstream open reading frames was found to interfere with induction of the ctc-lacZ fusion in response to glucose. Additional deletion analysis and complementation studies showed that orfU was required for full glucose induction of sigma B-dependent genes. orfU encodes a trans-acting, positive factor with significant sequence identity to the RsbX negative regulator of sigma B. On the basis of these results, we rename orfU as rsbU to symbolize the regulatory role of its product.