Transcriptional attenuation and differential mRNA stability in the regulation of the Escherichia coli melibiose operon.

The organization of the melibiose operon of Escherichia coli is promoter-melA-melB. The amount of the product (alpha-galactosidase) of the first gene (melA) is much larger than that of the product (melibiose permease) of the second gene (melB). Using the chloramphenicol acetyltransferase gene (cat) as reporter, we found that there was an element between melA and melB, which reduced the expression of the downstream gene, melB. This region contained a boxA-like sequence, which is known as a binding site for an attenuation factor, NusA. Northern hybridization analysis revealed that the ratio of melA mRNA and melAB mRNA was comparable with the ratio of the melA and melB products. We also found that the melA mRNA was about 3-fold more stable than the melAB mRNA. Experimental results obtained with a nusAts mutant suggested that the NusA protein is involved in the reduced expression of the melB gene. We conclude that the production ratio of alpha-galactosidase and melibiose permease is regulated at two levels: 1) transcription and 2) mRNA stability.