External-pH-Dependent expression of the maltose regulon and ompF gene in Escherichia coli is affected by the level of glycerol kinase, encoded by glpK

The expression of the maltose system in Escherichia coli is regulated at bo th transcriptional and translational levels by the pH of the growth medium (pHo). With glycerol as the carbon source, transcription of malT, encoding the transcriptional activator of the maltose regulon, is weaker in acidic m edium than in alkaline medium. malT transcription became high, regardless o f the pHo, when glycerol-3-phosphate or succinate was used as the carbon so urce. Conversely, malT expression was low, regardless of the pHo, when malt ose was used as the carbon source. The increase in malT transcription, asso ciated with the pHo, requires the presence of glycerol in the growth medium and the expression of the glycerol kinase (GlpK). Changes in the level of glpK transcription had a great effect on malT transcription. Indeed, a glpF KX promoter-down mutation has been isolated, and in the presence of this mu tation, malT expression was increased. When glpK was expressed from a high- copy-number plasmid, the glpK-dependent reduced expression of the maltose s ystem became effective regardless of the pHo. Analysis of this repression s howed that a malTp1 malTp10 promoter, which is independent of the cyclic AM P (cAMP)-cAMP receptor protein (CRP) complex, was no longer repressed by gl pFKX amplification. Thus, GlpK-dependent repression of the maltose system r equires the cAMP-CRP complex. We propose that the pHo may affect a complex interplay between GlpK, the phosphotransferase-mediated uptake of glucose, and the adenylate cyclase.