Metabolism of messenger RNA from the gal operon of Escherichia coli

Abstract

The size and metabolism of messenger RNA from the galactose ( gal) operon of Escherichia coli has been studied. The three known structural genes of the operon are about 1100 nucleotide pairs each and code for an epimerase ( E), transferase ( T) and kinase ( K) gene, in that order. (1) The full-length gal mRNA in both B and K12 strains is about 4500 nucleotides or 1200 longer than that predicted for these three genes. The transcription time to the end of the epimerase gene was estimated by the lag before appearance of transferase mRNA. These measurements are consistent with about 520 nucleotides of gal mRNA proximal to the transferase message. The position of the remaining additional nucleotides is not known. (2) The functional inactivation rate of each message was measured by following the decreasing rate of enzyme synthesis after a one-minute induction by fucose. The half-life for each message was: epimerase 1.0 minute, transferase 0.6 minute, and kinase 1.5 minutes. (3) Mass decay of gal mRNA, induced for one minute, was followed by hybridization to DNA from transducing λ gal phages. Messenger RNA from the operator-proximal half, operator-distal two-thirds or from the entire operon decays with a half-life of about 1.1 minutes. These data cannot determine whether or not mass decay of each message is proportional to its inactivation rate. (4) The size distributions of decaying gal mRNA were determined in sucrose gradients or polyacrylamide gels with time of decay. They show clearly that gal mRNA is attacked by endonucleolytic cleavage. Fragments of about 2200 nucleotide length accumulate and have the specificity of transferase-kinase mRNA. The rapid inactivation of transferase message may result from cleavage of the epimerase-transferase mRNA junction to generate these transferase-kinase pieces.