Abstract
Individualrrnoperons and their flanking regions have been analysed in a study of the molecular basis of ribotype variation in the seventh pandemic clone ofVibrio cholerae.The genome of an early isolate of the seventh pandemic clone had ninerrnoperons of which two were in tandem with otherrrnoperons. The site forBgll, the most discriminatory enzyme used for ribotyping, was found to be present in the 16S sequence of three of the operons of the earliest isolate. This site was observed to be gained or lost in specific operons in many later isolates, presumably by recombination, and this gave most of the ribotype variation. Additionalrrnrecombination events were uncovered by analysis of the 16S-23S intergenic spacers associated with each operon. Spacers of 431, 509, 607 and 711 bp were found. A total of at least eightrrnrecombination events were detected. Threerrnloci were primarily involved in this recombination, with four new forms generated from that in the early strains for operon B and two new forms each for operons C and G. In addition there was variation due to deletion of tandem operons. The frequency of recombination betweenrrnoperons was very high as there were nine new ribotypes found among 47 isolates sampled over the 33 year period of study. This means that any variation could undergo precise reversion by the same recombination event within the time frame covered by the study. Recombination betweenrrnoperons may be a factor in ribotype variation in all systems. The recombination observed is thought to be that which results in concerted evolution and the data give an indication of the rate involved.

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