The complete DNA sequence and analysis of R27, a large IncHI plasmid from Salmonella typhi that is temperature sensitive for transfer

Salmonella typhi, the causative agent of typhoid fever, annually infects 16 million people and kills 600 000 world wide. Plasmid-encoded multiple drug resistance in S. typhi is always encoded by plasmids of incompatibility gr oup H (IncH). The complete DNA sequence of the large temperature-sensitive conjugative plasmid R27, the prototype for the IncHI1 family of plasmids, h as been compiled and analyzed, This 180 kb plasmid contains 210 open readin g frames (ORFs), of which 14 have been previously identified and 56 exhibit similarity to other plasmid and prokaryotic ORFs. A number of insertion el ements were found, including the full Tn10 transposon, which carries tetrac ycline resistance genes. Two transfer regions, Tra1 and Tra2, are present, which are separated by a minimum of 64 kb. Homologs of the DNA-binding prot eins TIpA and H-NS that act as temperature-regulated repressors in other sy stems have been located in R27, Sequence analysis of transfer and replicati on regions supports a mosaic-like structure for R27. The genes responsible for conjugation and plasmid maintenance have been identified and mechanisms responsible for thermosensitive transfer are discussed.