Mutations that relieve nutritional repression of the Bacillus subtilis dipeptide permease operon

Abstract

The Bacillus subtilis dcU operon encodes a dipeptide transport complex that is induced rapidly as cells enter stationary phase and initiate sporulation. Expression of this operon in growing cells is repressed by glucose, by a mixture of amino acids, and by the AbrB protein. A genetic screen was devised to identify mutations that allow inappropriate expression from the dcL4 promoter during growth. These mutations resulted in increased dci4 transcription during growth in nutrient broth, in minimal amino acids medium, and in minimal glucose medium. Some of the mutations, called des (dci control site), were cloned and shown by sequence analysis to cluster near the start site of dci4 transcription. Primer extension and in vitro transcription analysis revealed that the dcs mutations did not create a new promoter. These mutations may therefore disrupt an operator site necessary for the binding of a negative regulator responsive to the nutritional state of the cell. The dcs mutant promoters were still subject to AbrB control, suggesting that the dci4 operon is regulated by at least two proteins, AbrB and a nutritionally responsive regulator. The gene(s) for the putative nutritional regulator may be defined by the cod (control ofdciA) mutations, which appeared to relieve amino acid and glucose repression ofdci4 by altering a diffusible factor. An abrB cod double mutant exhibited high-level expression ofdci4 during exponential growth phase.In response to conditions of carbon, nitrogen, or phosphorus limitation, Bacillus subtilis cells undergo a primitive metamorphosis to produce an environmentally resistant spore (19), but the mechanism by which initiation of sporulation is triggered by nutrient depletion is poorly understood (37). The expression of many genes induced as the cells enter stationary phase is known to be regulated by carbon and nitrogen sources (9) and to require the action of various signal transduction systems and transcriptional regulators (for reviews, see references 15 and 36). The relationship between these regulators and the nutritional response is obscure, however. Some genes subject to such regulation, such as spoVG (12), are required for sporulation, but others, such as aprE (8), amyE (25), and gsiA (23), are not. The hut (histidine utilization) operon (1, 26) and the citB gene (5, 10), which encodes aconitase, are also induced early in stationary phase and are repressed independently by a mixture of amino acids and by rapidly metabolizable carbon sources. However, the molecular mechanisms of glucose repression and amino acid repression in B. subtilis remain elusive. Our studies have focused on the regulation of the dci4 (decoyinine induced [21]) operon as a model system for understanding nutritional control phenomena that occur at the onset of Bacillus development.In previous work, we showed that the dci4 operon encodes a dipeptide transport system that is dispensable for sporulation but may assist the cells in adapting to poor nutrient conditions (20). This operon is transcribed at a very low lev...

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The OhrR repressor senses organic hydroperoxides by reversible formation of a cysteine-sulfenic acid derivative

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