AUTOGENOUS REGULATION OF ETHANOLAMINE UTILIZATION BY A TRANSCRIPTIONAL ACTIVATOR OF THE EUT OPERON IN SALMONELLA-TYPHIMURIUM

The genes required for use of ethanolamine as a carbon and nitrogen source are encoded by a single operon (eut) whose expression is induced by the simultaneous presence of both ethanolamine and cobalamin (vitamin B-12). The action of B-12 as an inducer of this operon reflects the fact that this cofactor is required by the degradative enzyme ethanolamine lyase (eutBC). The eutR gene encodes a protein that activates transcription of the eut operon in response to the simultaneous presence of B-12 and ethanolamine. The eutR gene is expressed by a weak constitutive promoter activity (P(II)) and by the main regulated promoter (P(I)). Because it is encoded within the operon that it activates, the EutR protein controls its own production. Initial induction of the eut operon by ethanolamine plus B-12 causes an increase in expression of the eutR gene; this increase acts as part of a positive feedback loop that is required for maximal operon expression. Because of this mode of regulation, constitutive regulatory mutations, described here, include mutations that generate new internal promoters and thereby increase the basal level of eutR gene expression. In mutants with an increased level of activator protein, each inducer (B-12 or ethanolamine), presented singly, is sufficient for partial operon induction.