RECOMBINATION BETWEEN CHROMOSOMAL IS200 ELEMENTS SUPPORTS FREQUENT DUPLICATION FORMATION IN SALMONELLA-TYPHIMURIUM

Spontaneous tandem chromosomal duplications are common in populations of Escherichia coli and Salmonella typhimurium. They range in frequenc y for a given locus from 10(-2) to 10(-4) and probably form by RecA-de pendent unequal sister strand exchanges between repetitive sequences i n direct order. Certain duplications have been observed previously to confer a growth advantage under specific selective conditions. Tandem chromosomal duplications are unstable and are lost at high frequencies , representing a readily reversible source of genomic variation. Six c opies of a small mobile genetic element IS200 are evenly distributed a round the chromosome of S. typhimurium strain LT2. A survey of 120 ind ependent chromosomal duplications (20 for each of six loci) revealed t hat recombination between IS200 elements accounted for the majority of the duplications isolated for three of the loci tested. Duplications of the his operon were almost exclusively due to recombination between repeated IS200 elements. These data add further support to the idea t hat mobile genetic elements provide sequence repeats that play an impo rtant role in recombinational chromosome rearrangements, which may con tribute to adaptation of bacteria to stressful conditions.