Synthesis and degradation of the mRNA of the Tn21 mer operon

The mercury resistance locus encoded by Tn21 on the monocopy IncFII plasmid R100 (merTn21) consists of a metal-responsive activator/repressor, merR, which controls initiation of a polycistronic message that includes genes for the uptake (merTPC) and reduction (merA) of Hg2+ and merD, which may also play a minor regulatory role. Comparison of the relative abundance of the 5' and 3' ends of the merTPCAD transcript revealed a strong transcriptional gradient in the operon, consistent with previous observations of lower relative abundance of the more promoter-distal gene products. In vivo mRNA degradation rates varied only slightly for the different genes: however, the rates of mRNA synthesis varied considerably from the beginning to the end of the operon. Specifically, mRNA corresponding to the promoter-proximal genes, merTPC, achieved a maximum in vivo synthesis rate between 60 and 120 seconds after induction; this rate was maintained for approximately ten minutes. In contrast, the synthesis rates of mRNA corresponding to the promoter-distal genes merA and merD, were initially fivefold lower than the rates of the promoter-proximal genes for the first five minutes after induction, and then rose gradually to approximately 50% of the merTPC synthesis rates. These data suggested that early after induction only 20% of the transcripts initiating at merT proceed beyond merC. At later times after induction approximately 50% of the transcripts proceed beyond merC. Nuclease end mapping did not reveal any discrete termination events in the merPCA region, thus, premature termination may occur at many sites.