The histidine utilization (hut) locus of Pseudomonas fluorescens SBW25 confers the ability to utilize histidine as a sole carbon and nitrogen source. Genetic analysis using a combination of site-directed mutagenesis and chromosomally integrated lacZ fusions showed the hut locus to be composed of 13 genes organized in 3 transcriptional units: hutF, hutCD, and 10 genes from hutU to hutG (which includes 2 copies of hutH, 1 of which is nonfunctional). Inactivation of hutF eliminated the ability to grow on histidine, indicating that SBW25 degrades histidine by the five-step enzymatic pathway. The 3 hut operons are negatively regulated by the HutC repressor with urocanate (the first intermediate of the histidine degradation pathway) as the physiological inducer. 5'-RACE analysis of transcriptional start sites revealed involvement of both sigma(54) (for the hutU-G operon) and sigma(70) (for hutF); the involvement of sigma(54) was experimentally demonstrated. CbrB (an enhancer binding protein for sigma(54) recruitment) was required for bacterial growth on histidine, indicating positive control of hut gene expression by CbrB. Recognition that a gene (named hutD) encoding a widely distributed conserved hypothetical protein is transcribed along with hutC led to analysis of its role. Mutational and gene fusion studies showed that HutD functions independently of HutC. Growth and fitness assays in laboratory media and on sugar beet seedlings suggest that HutD acts as a governor that sets an upper bound to the level of hut activity.