Complete DNA sequence and comparative analysis of the 50-kilobase virulence plasmid of Salmonella enterica serovar choleraesuis

The complete nucleotide sequence of pKDSC50, a large virulence plasmid from Salmonella enterica serovar Choleraesuis strain RF-1, has been determined. We identified 48 of the open reading frames (ORFs) encoded by the 49,503-b p molecule. pKDSC50 encodes a known virulence associated operon, the spy op eron, which is composed of genes essential for systemic infection by nontyp hoidal Salmonella. Analysis of the genetic organization of pKDSC50 suggests that the plasmid is composed of several virulence-associated genes, which include the sppRABCD genes, plasmid replication and maintenance genes, and one insertion sequence element. A second virulence associated region includ ing the pef (plasmid-encoded fimbria) operon and I ck (resistance to comple ment killing) gene, which has been identified on the virulence plasmid of S . enterica serovar Typhi murium, was absent. Two different replicon regions , similar to the RepFIIA and RepFIB replicons, were found. Both showed high similarity to those of the pO157 plasmid of enterohemorrhagic Escherichia coli O157:H7 and the enteropathogenic E. coli (EPEC) adherence factor plasm id harbored by EPEC strain B171 (O111:NM), as well as the virulence plasmid s of Salmonella serovars Typhimurium and Enteritidis. Comparative analysis of the nucleotide sequences of the 50-kb virulence plasmid of serovar Chole raesuis and the 94-kb virulence plasmid of serovar Typhimurium revealed tha t 47 out of 48 ORFs of the virulence plasmid of serovar Choleraesuis are hi ghly homologous to the corresponding ORFs of the virulence plasmid of serov ar Typhimurium, suggesting a common ancestry.