CONTROL OF THE EXPRESSION OF THE MANXYZ OPERON IN ESCHERICHIA-COLI - MLC IS A NEGATIVE REGULATOR OF THE MANNOSE PTS

The manXYZ operon of Escherichia coil encodes a sugar transporter of t he phosphoenol pyruvate (PEP)-dependent phosphotransferase system, whi ch is capable of transporting many sugars, including glucose, mannose and the aminosugars, glucosamine and N-acetylglucosamine. Transcriptio n of manX is strongly dependent on cyclic AMP (cAMP)/cAMP receptor pro tein (CAP). A cAMP/CAP binding site is located at -40.5, and activatio n by cAMP/CAP is shown to be typical of a class II promoter. The 5' en d of a transcript, potentially encoding two proteins, is expressed div ergently from the manXYZ operon. Previously, two binding sites for the NagC repressor were detected upstream of manX, but a mutation in nagC has very little effect on manX expression. However, a mutation in the mle gene, encoding a homologue of nagC, results in a threefold derepr ession of manX expression, suggesting that this protein is a more impo rtant regulator of manX expression than NagC. The Mlc protein binds to the NagC operators, binding preferentially to the promoter-proximal o perator. Plasmids overproducing either the NagC protein or the Mlc pro tein repress the expression of manX, but the effect of the Mlc protein is stronger. The mle gene is shown to be allelic with the previously characterized dgsA mutation affecting the mannose phosphoenolpyruvate- dependent phosphotransferase system (PTS).