ACTIVATION OF THE BACILLUS-SUBTILIS-HUT OPERON AT THE ONSET OF STATIONARY GROWTH-PHASE IN NUTRIENT SPORULATION MEDIUM RESULTS PRIMARILY FROM THE RELIEF OF AMINO-ACID REPRESSION OF HISTIDINE TRANSPORT

During growth of Bacillus subtilis in nutrient sporulation medium cont aining histidine (DSM-His medium), the expression of histidase, the fi rst enzyme in the histidine-degradative pathway (hut), is derepressed 40- to 200-fold at the onset of stationary phase. To identify the gene products responsible for this regulation, histidase expression was ex amined in various hut regulatory mutants as well as in mutants defecti ve in stationary-phase gene regulation. Histidase expression during gr owth in DSM-His medium was significantly altered only in a strain cont aining the hutC1 mutation. The hutC1 mutation allows the hut operon to be expressed in the absence of its inducer, histidine. During logarit hmic growth in DSM-His medium, histidase levels were 25-fold higher in the HutC mutant than in wild-type cells. Moreover, histidase expressi on in the HutC mutant increased only four- to eightfold after the end of exponential growth in DSM-His medium. This suggests that histidine transport is reduced in wild-type cells during exponential growth in D SM-His medium and that this reduction is largely responsible for the r epression of hut expression in cells growing logarithmically in this m edium. Indeed, the rate of histidine uptake in DSM-His medium was four fold lower in exponentially growing cells than in stationary-phase cel ls. The observation that the degradation of histidine is inhibited whe n B. subtilis is growing rapidly in medium containing a mixture of ami no acids suggests that a hierarchy of amino acid utilization may be pr esent in this bacterium.