pH-regulated gene expression in Salmonella: genetic analysis of aniG and cloning of the earA regulator.

The recently described aniG locus exhibits a series of unique regulatory features. The gene is exogenously coinduced by acid and D-mannose, its expression is maximal under anaerobiosis, and the system is regulated in an unusual manner by cyclic AMP. The external acid regulatory locus, earA, is a repressor protein that mediates the acid and mannose control of aniG. The earA locus was cloned and found to code for a 33K protein associated with membrane- and soluble fractions. A second locus, earB, was located immediately upstream from earA. The earB locus or its product interferes with the repression of aniG by EarA. Mutations in crp and cya were found to prevent transcription of aniG but only in an earA+ background. Analysis of an earA-cat fusion established that crp does not affect earA expression. While the physiological role of aniG/earA is unclear, this system serves as a model for external pH-regulated gene expression. The present data indicate that it is used to sense the presence of mannose in an acidic extracellular environment. This is particularly intriguing in that the system is not involved in the utilization of mannose as a carbon source.