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Journal of Bacteriology, February 2002, p. 666-671, Vol. 184, No. 3
0021-9193/01/$04.00+0     DOI: 10.1128/JB.184.3.666-671.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Nested Evolution of a tRNALeu(UAA) Group I Intron by both Horizontal Intron Transfer and Recombination of the Entire tRNA Locus

Knut Rudi,1,2* Tonje Fossheim,2 and Kjetill S. Jakobsen2

MATFORSK, Norwegian Food Research Institute, 1430 Ås,1 Division of General Genetics, Department of Biology, University of Oslo, Blindern, 0315 Oslo, Norway2

Received 15 June 2001/ Accepted 29 October 2001

The origin and evolution of bacterial introns are still controversial issues. Here we present data on the distribution and evolution of a recently discovered divergent tRNALeu(UAA) intron. The intron shows a higher sequence affiliation with introns in tRNAIle(CAU) and tRNAArg(CCU) genes in {alpha}- and ß-proteobacteria, respectively, than with other cyanobacterial tRNALeu(UAA) group I introns. The divergent tRNALeu(UAA) intron is sporadically distributed both within the Nostoc and the Microcystis radiations. The complete tRNA gene, including flanking regions and intron from Microcystis aeruginosa strain NIVA-CYA 57, was sequenced in order to elucidate the evolutionary pattern of this intron. Phylogenetic reconstruction gave statistical evidence for different phylogenies for the intron and exon sequences, supporting an evolutionary model involving horizontal intron transfer. The distribution of the tRNA gene, its flanking regions, and the introns were addressed by Southern hybridization and PCR amplification. The tRNA gene, including the flanking regions, were absent in the intronless stains but present in the intron-containing strains. This suggests that the sporadic distribution of this intron within the Microcystis genus cannot be attributed to intron mobility but rather to an instability of the entire tRNALeu(UAA) intron-containing genome region. Taken together, the complete data set for the evolution of this intron can best be explained by a model involving a nested evolution of the intron, i.e., wherein the intron has been transferred horizontally (probably through a single or a few events) to a tRNALeu(UAA) gene which is located within a unstable genome region.

* Corresponding author. Mailing address: MATFORSK, Norwegian Food Research Institute, Osloveien 1, 1430 Ås, Norway. Phone: 47-64-97-02-66. Fax: 47-64-97-03-33. Email: knut.rudi{at}matforsk.no.

Journal of Bacteriology, February 2002, p. 666-671, Vol. 184, No. 3
0021-9193/01/$04.00+0     DOI: 10.1128/JB.184.3.666-671.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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