Synthesis and degradation of the mRNA of the Tn21 mer operon

• Published in: Journal of molecular biology Vol. 225; no. 2; pp. 251 - 259
• Main Authors: Gambill, B.Diane, Summers, Anne O.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 20.05.1992; Elsevier
• Subjects: attenuation; Bacteriology; Base Sequence; Biological and medical sciences; DNA Transposable Elements - genetics; Drug Resistance, Microbial - genetics; Escherichia coli - genetics; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation, Bacterial - genetics; Genetics; Mercury - pharmacology; mercury resistance; Microbiology; Molecular Sequence Data; Nucleic Acid Hybridization; Operon - genetics; R Factors - genetics; RNA, Messenger - biosynthesis; RNA, Messenger - metabolism; Transcription, Genetic - genetics; transcriptional polarity; mercury resistance; attenuation; transcriptional polarity; Messenger RNA; Chemical resistance; Transcription; Operon; Escherichia coli; Transposon; Inheritance; Bacteria; Mercury; Enterobacteriaceae
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/0022-2836(92)90919-B

The mercury resistance locus encoded by Tn21 on the monocopy IncFII plasmid R100 ( mer Tn21) 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 Hg 2+ 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.