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 i n the structural genes of the L-rhamnose regulon of Escherichia coli h as permitted identification of the open reading frames corresponding t o rhaB, rhaA, and rhaD. The deduced amino acid sequences gave a 425-am ino-acid polypeptide corresponding to rhamnulose kinase for rhaB, a 40 0-amino-acid polypeptide corresponding to rhamnose isomerase for rhaA, and a 274-amino-acid polypeptide corresponding to rhamnulose-1-phosph ate aldolase for rhaD. Transcriptional fusions of the three putative p romoter regions to lacZ showed that only the rhaB leader region acted as a promoter, as indicated by the high beta-galactosidase activity in duced by rhamnose, while no significant activity from the rhaA and rha D constructions was detected. The rhaB transcription start site was ma pped to -24 relative to the start of translation. Mutations in the cat abolic genes were used to show that L-rhamnose may directly induce rha BAD transcription.