A cryptic fimbrial gene in Serratia marcescens

The gene coding for the mannose-sensitive hemagglutinating fimbriae in Serratia marcescens US5 was cloned into Escherichia coli K4 with a cosmid vector system. One of the transformants, US5-1, expressed two morphologically distinct fimbriae, one that was 5-nm wide and one that was 3-nm wide. The latter fimbria was morphologically and serologically indistinguishable from that of strain US5. Genetic analysis of transformant US5-1 showed that the gene responsible for the 5-nm-wide fimbriae was located more than 10 kilobases away from the gene responsible for the 3-nm-wide fimbriae. The molecular size of the subunits of these two fimbriae, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were 19 kilodaltons for the 3-nm-wide fimbriae and 20 kilodaltons for the 5-nm-wide fimbriae. Serologically, the 5-nm-wide fimbriae did not cross-react with monoclonal or polyclonal antibodies raised against the mannose-sensitive hemagglutinating fimbriae of strain US5. Strain EL101, which expressed only the 5-nm-wide fimbriae, did not agglutinate chicken or human erythrocytes. These experimental results suggest that the gene for the 5-nm-wide fimbriae is cryptic in strain US5 and is expressed in E. coli K4 only after it is moved by transformation.