Isolation and molecular characterization of spontaneously occurring cytolysin-negative mutants of Actinobacillus pleuropneumoniae serotype 7.

Actinobacillus pleuropneumoniae serotype 7 strains are shown to spontaneously lose cytolytic activity with a frequency of approximately 10(-4). The phenotypic change is associated with the loss of approximately 8.5 kbp of chromosomal DNA. A genomic fragment encoding the cytolysin and its flanking sequences was cloned and characterized. Also, the corresponding truncated fragment was cloned from a spontaneous mutant. Comparison of the two clones allowed the definition of the excision site. The ends of the excised fragment are composed of 1,201 bp long direct identical repeats, possibly facilitating the genotypic change by homologous recombination. In accordance with this hypothesis, one repeat is conserved in the spontaneous mutant. Each repeat contains one open reading frame preceded by a Shine-Dalgarno consensus sequence, and the ends of each repeat contain 26-bp complementary sequences with four mismatches.