ANALYSIS OF THE GENETIC-POLYMORPHISM BETWEEN 3 STREPTOCOCCUS-THERMOPHILUS STRAINS BY COMPARING THEIR PHYSICAL AND GENETIC ORGANIZATION

The physical maps of Streptococcus thermophilus CNRZ368 and NST2280 st rains were constructed by analysing PFGE patterns obtained with the lo w-frequency-cutting enzymes SmaI, BssHII and SfiI. Their chromosomes a re 1864 and 1840 kb circular molecules, respectively. Comparison of th eir physical maps with that of the reference A054 strain revealed a re latively conserved organization of the restriction sites. Three variab le regions were detected with the map of CNRZ368 whereas 15 were found with the map of NST2280. To construct the genetic maps, probes corres ponding to 10 single-copy genes, the rm genes and the insertion sequen ces IS1191, IS981 and ISS1 were hybridized to Southern blots of chromo somal DNA digested with the different mapping enzymes. Comparison of t he genetic maps of the three strains showed a conserved location of th e mapped single-copy genes. However, six rm loci were present in the c hromosome of A054 and CNRZ368 whereas five were present in the NST2280 chromosome. A polymorphism was also found in the copy number of the i nsertion sequences between the three strains.