Summary: pBF4 is a 41 kb conjugative R-plasmid that confers MLS (macrolide-lincosamide-streptogramin B) resistance in spp. To identify pBF4 genes governing conjugation, recombinational mutagenesis using a suicide vector carrying fragments of the pBF4 plasmid was employed. One of six independent insertion mutants of pBF4 isolated using this method was found to be conjugation-deficient. Nucleotide sequence analysis around the insertion site on this plasmid revealed a 2.8 kb ORF that encoded a putative 110 kDa protein. A corresponding protein was observed when a 12 kb DNA fragment containing this ORF was used to program an transcription-translation system. Both the ORF and the predicted protein were novel when compared to available database sequences. This gene was designated (). Polyclonal rabbit antibodies that recognized a sub-sequence polypeptide of BctA reacted with a 55 kDa protein in Western blot analysis using a total protein extract from containing pBF4. The protein was not present in a strain containing the conjugation-deficient insertion mutant of pBF4. The 55 kDa protein was associated with the membrane fraction of Although the cellular and biochemical basis of -promoted conjugation remains unknown, this work demonstrates the existence of a heretofore unrecognized gene in bacterial conjugation, and the mutagenesis system used provides the means to isolate and characterize other genes involved in conjugal transfer in spp.


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