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The polycistronic mRNA of the histidine operon is subject to a processing event that generates a rather stable transcript encompassing the five distal cistrons. The molecular mechanisms by which such a transcript is produced were investigated in Escherichia coil strains carrying mutations in several genes for exo-and endonucleases. The experimental approach made use of S1 nuclease protection assays on in vivo synthesized transcripts, site-directed mutagenesis and construction of chimeric plasmids, dissection of the processing reaction by RNA mobility retardation experiments, and in vitro RNA degradation assays with cellular extracts. We have found that processing requires (1) a functional endonuclease E; (2) target site(s) for this activity in the RNA region upstream of the 5' end of the processed transcript that can be substituted by another well-characterized me-dependent cleavage site; (3) efficient translation initiation of the first cistron immediately downstream of the 5' end; and (4) a functional endonuclease P that seems to act on the processing products generated by ribonuclease E. This is the first evidence that ribonuclease P, an essential ribozyme required for the biosynthesis of tRNA, may also be involved in the segmental stabilization of a mRNA.[Key Words: Escherichia coli; Salmonella typhimurium; his operon; mRNA processing; translation; RNase E; RNase P] Received June 2, 1994; revised version accepted October 18, 1994. mRNA decay in bacteria is mediated by the coordinated action of exonucleases and endonucleases (for review, see Petersen 1992). Polynucleotide phosphorylase (PNPase) and ribonuclease II (RNase II) are the two major 3'--* 5' exonucleases involved in mRNA turnover (Donovan and Kushner 1986). Endoribonuclease III (RNase III) does not affect bulk mRNA stability (Babitzke et al. 1993) and is mostly involved in processing of the 30S rRNA precursor even though RNase III can initiate the functional decay of a few specific mRNAs (Court 1993). Endoribonuclease E (RNase E)was initially characterized as the enzyme that processes the precursor of 5S rRNA (Ghora and Apirion 1978). RNase E is the only endoribonuclease implicated thus far in general mRNA turnover (Arraiano et al. 1988;Mudd et al. 1990b;Babitzke and Kushner 1991). Many mRNAs were shown to contain sites whose cleavage was abolished in strains harboring temperature-sensitive rne mutations at the nonpermissive temperature (Kokoska et al. 1990;Mudd et al. 1990a;Gross 1991 1991; Patel and Dunn 1992;Arraiano et al. 1993;Klug 1993;Mudd and Higgins 1993;Carpousis et al. 1994). rne-dependent cleavages might produce relatively stable processed transcripts, such as the T4 gene 32 (Mudd et al. 1988) and the dicF (Faubladier et al. 1990) and the ghX mRNAs (Brunet al. 1990). In different studies ribonuclease P (RNase P) and other nucleases responsible for stable RNA processing did not seem to be involved in mRNA degradation (Deutscher 1988).We have previously identified in Salmonella typhimurium a rather stable 3900-nucleotide-long mRNA molecule...

We have characterized a major processed species of mRNA in the his operon of Salmonella typhimurium. In vivo and in vitro analyses of the his transcripts from wild-type and mutant strains using S1 nuclease protection assays, measurements of RNA stability, deletion mapping, gel retardation, and in vitro translation assays demonstrate that the distal portion of the polycistronic his mRNA is processed, resulting in increased stability. The processing event requires an upstream cis-acting element and translation of the cistron immediately downstream of the 5' end of the processed species. The cistrons contained in this segment are also independently transcribed from an internal promoter which is maximally active in the absence of readthrough transcription from the primary promoter.

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