Lawrence Berkeley National Laboratory

Yersinia pestis, the causative agent of bubonic and pneumonic plague, has undergone detailed study at the molecular level. To further investigate the genomic diversity among this group and to help characterize lineages of the plague organism that have no sequenced members, we present here the genomes of two isolates of the "classical" Antiqua biovar, strains Antiqua and Nepal516. The genomes of Antiqua and Nepal516 are 4.7 Mb and 4.5 Mb and encode 4,138 and 3,956 open reading frames respectively. Though both strains belong to one of the three classical biovars, they represent separate lineages defined by recent phylogenetic studies. We compare all five currently sequenced Y. pestis genomes and the corresponding features in Y. pseudotuberculosis. There are strain-specific rearrangements, insertions, deletions, single nucleotide polymorphisms and a unique distribution of insertion sequences. We found 453 single nucleotide polymorphisms in protein coding regions, which were used to assess evolutionary relationships of these Y. pestis strains. Gene reduction analysis revealed that the gene deletion processes are under selective pressure and many of the inactivations are probably related to the organism s interaction with its host environment. The results presented here clearly demonstrate the differences between the two Antiqua lineages and support the notion that grouping Y. pestis strains based strictly on the classical definition of biovars (predicated upon two biochemical assays) does not accurately reflect the phylogenetic relationships within this species. Comparison of four virulent Y. pestis strains with the human-avirulent strain 91001 provides further insight into the genetic basis of virulence to humans.