title: Synthesis and degradation of the mRNA of the Tn21 mer operon.
abstract: 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.
authors.affiliation: Department of Microbiology, University of Georgia, Athens 30602.
authors.forename: B D
authors.initials: BD
authors.lastname: Gambill
authors.forename: A O
authors.initials: AO
authors.lastname: Summers
mesh_terms: D001483:Base Sequence; D004251:DNA Transposable Elements / Q000235:genetics*; D004352:Drug Resistance, Microbial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial / Q000235:genetics; D008628:Mercury / Q000494:pharmacology*; D008969:Molecular Sequence Data; D009693:Nucleic Acid Hybridization; D009876:Operon / Q000235:genetics*; D011815:R Factors / Q000235:genetics; D012333:RNA, Messenger / Q000096:biosynthesis / Q000378:metabolism*; D014158:Transcription, Genetic / Q000235:genetics
chemical_list: D004251:DNA Transposable Elements; D012333:RNA, Messenger; D008628:Mercury