sentence: Y. kristensenii ATCC 33638T had an identical 13 gene tad locus in the same position , but the nucleotide sequence identity of the region to Y. enterocolitica 8081 was uniformly lower than that found for the rest of the genome , suggesting there had been either a gene conversion event replacing the tad locus with a set of new alleles in the recent history of Y. kristensenii or Y. enterocolitica or the locus was under very high positive selective pressure .