Sequencing and characterization of a gene cluster encoding the enzymes for L-rhamnose metabolism in Escherichia coli.

The sequencing of the EcoRI-HindIII fragment complementing mutations in the structural genes of the L-rhamnose regulon of Escherichia coli has permitted identification of the open reading frames corresponding to rhaB, rhaA, and rhaD. The deduced amino acid sequences gave a 425-amino-acid polypeptide corresponding to rhamnulose kinase for rhaB, a 400-amino-acid polypeptide corresponding to rhamnose isomerase for rhaA, and a 274-amino-acid polypeptide corresponding to rhamnulose-1-phosphate aldolase for rhaD. Transcriptional fusions of the three putative promoter regions to lacZ showed that only the rhaB leader region acted as a promoter, as indicated by the high beta-galactosidase activity induced by rhamnose, while no significant activity from the rhaA and rhaD constructions was detected. The rhaB transcription start site was mapped to -24 relative to the start of translation. Mutations in the catabolic genes were used to show that L-rhamnose may directly induce rhaBAD transcription.