High-frequency deletion involving closely spaced rRNA gene sets in Streptococcus thermophilus

Homologous rDNA probes were used to study the number of rRNA genes in a five-generation genealogy of Streptococcus thermophilus CNRZ368. Whereas the CNRZ368 strain contained six rRNA loci, four independant mutants with five rrn loci were obtained. The deletion frequency was 5 × 10−2. Molecular analysis provided identical hybridization patterns for all the deletion mutants. The deletion was shown to occur within the two close rRNA loci, rrnD and rrnE, probably due to a homologous recombination event and to give rise to a hybrid rrnD/E locus.