Contributions of traT and iss genes to the serum resistance phenotype of plasmid ColV2‐K94

A genetic determinant for serum resistance, designated iss , has been found previously on the colicinogenic plasmid ColV2‐K94. In this work we have identified a second serum resistance gene, traT , on ColV2‐K94. The serum resistance mediated by derivatives of ColV2‐K94 was due to presence of one or both of the iss and traT genes. Plasmid pWS12 (TraT + Iss + ) contained the kanamycin (Km) resistance transposon Tn 903 inserted near the origin of replication of ColV2‐K94, and plasmids pWS15 (TraT + ), pWS16 (TraT + ) and pWS18 (TraT + Iss + ) were deletion derivatives of pWS12 constructed in vitro and in vivo. pWS12 and pWS18 conferred a 20‐fold increase in relative resistance to 20% guinea pig serum when introduced into the serum‐susceptible, genetically defined recA strain of Escherichia coli K‐12, AB2463. Plasmids pWS15 and pWS16, from which iss had been deleted, still conferred 5‐fold increases in relative resistance on AB2463. The level of resistance conferred on this strain by the antibiotic resistance plasmid R100–1 (which expresses the traT serum resistance gene) was comparable to that of plasmids pWS15 and pWS16. The 25‐kDa traT gene product was identified by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) analysis of the outer membrane proteins of strain AB2463 carrying ColV2‐K94. This protein cross‐reacted immunologically with the traT protein expressed by F or R100–1. Our results indicated that both traT and iss are capable of mediating serum resistance in ColV2‐K94.