CHROMOSOMAL GENETIC INSTABILITY IN STREPTOCOCCUS-THERMOPHILUS

The thermophilic and homofermentative lactic acid bacterium Streptococ cus thermophilus has a relatively small genome (almost-equal-to 1.8 Mb ). Nevertheless, several loci which were unstable or exhibiting sequen ce polymorphism were found. The rRNA gene number varies according to t he strain. Whereas the CNRZ368 strain contains 6 rrn loci, 5% of its o ffspring only have 5. This alteration was shown to result from deletio ns occurring within the 2 close rRNA loci, rrnD and rrnE. Deletions ga ve rise to hybrid rrnD/E loci and probably originated from homologous recombination events. Intraspecific genetic polymorphism also affects the locus varA. The cloned 121 fragment belongs to the varA locus and reveals the various structures of this locus. Whereas some strains car ry the whole 121 fragment, some others lack either its right side or i ts left side. Both deletions and insertions of various length could re sult in such a situation. Moreover, evidence was provided that part of the 121 right side is repeated and dispersed in the genome. Another s ystem of genetic instability affects colony morphology. Four variants of Streptococcus thermophilus CNRZ368, differing in size, shape and op acity of colonies have been isolated. Switches between phenotypes occu r at different frequencies (10(-3) to 10(-1)). The 4 genotypes are dif ferent, as demonstrated by the 4 characteristic Xhol patterns of the 4 variants. In the case of the NST1403 strain, a CNRZ368 subclone exhib iting the ring phenotype, the mutation was shown to be either an inser tion or a duplication. Thus S thermophilus genomic instability results from various molecular mechanisms, all of them leading to intraspecif ic polymorphism.