title: Salmonella typhimurium LT2 possesses three distinct 23S rRNA intervening sequences.
abstract: The rrl genes for 23S rRNA of Salmonella typhimurium LT2 are known to carry intervening sequences (IVSs) at two sites, helix-25 and helix-45, which are excised by RNase III during rRNA maturation, resulting in rRNA which is fragmented but nevertheless functional. We isolated DNA fragments containing the seven rrl genes from BlnI, I-CeuI, and SpeI genomic digests following pulsed-field gel electrophoresis and used these DNA fragments as templates for PCRs utilizing primers upstream and downstream of helix-25 and helix-45. Variance in amplicon length and cycle sequencing indicated that rrlG and rrlH have IVSs in helix-25 of approximately 110 bp which are only 56% identical. rrnA, rrnB, rrnC, rrnD, rrnE, and rrnH have IVSs of approximately 90 bp in helix-45, and all have the same nucleotide sequence. Twenty-one independent wild-type strains of S. typhimurium from Salmonella Reference Collection A were analyzed for IVSs by using PCRs with genomic DNAs and by denaturing agarose electrophoresis of RNAs. Many strains resemble LT2, but some have no IVSs in helix-25 and others have IVSs in helix-45 in all seven rrl genes. However, the IVSs in individual wild-type lines are relatively stable, for several LT2 isolates separated over many years by many single-colony isolations are indistinguishable from one another, with the exception of line LB5010, which differs by one helix-25 IVS. We postulate that IVSs have entered strain LT2 by three independent lateral-transfer events and that the IVS in helix-45 was dispersed to and maintained in the same sequence in six of the seven rrl genes by the mechanism of gene conversion.
authors.affiliation: Salmonella Genetic Stock Centre, Uniersity of Calgary, Alberta, Canada. nmattata@acs.ucalgary.ca
authors.forename: N R
authors.initials: NR
authors.lastname: Mattatall
authors.forename: K E
authors.initials: KE
authors.lastname: Sanderson
mesh_terms: D001483:Base Sequence; D004275:DNA, Ribosomal; D005798:Genes, Bacterial; D007438:Introns; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D012338:RNA, Ribosomal, 23S / Q000235:genetics*; D012486:Salmonella typhimurium / Q000235:genetics*
chemical_list: D004275:DNA, Ribosomal; D012338:RNA, Ribosomal, 23S