Mapping of the pin locus coding for a site-specific recombinase that causes flagellar-phase variation in Escherichia coli K-12.

Although the vh2 mutation almost entirely prevents phase variation in Salmonella spp., an Escherichia coli strain that carried the Salmonella H1 and H2 region, including the vh2 mutation, showed phase variation. From this strain, EJ1076, a number of mutants defective in phase variation were isolated, and the symbol pin was assigned to their mutant gene. The pin locus was mapped between purB and trp near purB by interrupted matings using Tn10 sites inserted near pin. The locus was cotransduced with purB by P1 vir at a frequency of around 0.33. All the mutations tested were clustered at this locus. Three E. coli K-12 strains probably derived via different lines from the wild type have been tested for the presence of pin+ by introducing the two Salmonella H regions; two were pin+, and one was a pin mutant.