The genome structures of a temperate Lactobacillus phage, ~b FSW, and its virulent mutants, ~b FSVs, were examined by restriction, heteroduplex and nucleotide-sequence analyses. The results showed that two out of three ~b FSVs had the same 1.3 kbp insertion (designated as ISL1) at different positions in the ~bFSW sequence. ISL1 was 1,256 bp long and contained at least two long open reading frames of 279 and 822 bases on one strand. Inverted repeats were found at the termini of the ISL1 which was bracketed by 3 bp direct repeats of the ~bFSW sequence. From this evidence, we concluded that ISL1 was a transposable element in Lactobacillus casei. the ~bFSV-A genome which is indistinguishable from that of ~bFSW (Fig. 1). (2) In the enlarged regions the genomes of both qIFSV-B and ~bFSV-C had incorporated DNA sequences derived from a portion of the host chromosome.
(3) The enlargements of the ~b FSV genomes were not caused by illegitimate excision of the ~b FSW prophage, since the enlarged region was more than 7 kbp apart from the prophage attachment site on the host chromosome (Shimizu-Kadota and Tsuchida 1984; Shimizu-Kadota et al. 1984).
In this study, we have analysed the DNA structures of the ~bFSW and ~bFSV genomes in detail and found that the enlargement of both ~bFSV genomes was due to the insertion of a 1.3 kbp transposable element designated as ISL1 (previously denoted as the V-element).