THE DIVERSITY OF ALLELES AT THE HSD LOCUS IN NATURAL-POPULATIONS OF ESCHERICHIA-COLI

In enteric bacteria three discrete families of type I restriction and modification systems (IA, IB and ID) are encoded by alleles of the ser B-linked hsd locus. Probes specific for each of the three familes were used to monitor the distribution of related systems in 37 of the 72 w ild-type Escherichia coli strains comprising the ECOR collection. All 25 members of group A in this collection were screened; 12 were probe- positive, nine have hsd genes in the W family, true in the IB and one in the ID. Twelve strains, representing all groups other than A, were screened; five were probe-positive, one has hsd genes in the IA family , one in the IB and three in the ID. The type ID genes are the first r epresentatives of this family in E, coli, the probe-negative strains c ould have alternative families of hsd genes. The type IA and IB system s added at least five new specificites to the five already identified in natural isolates of E. coli. The distribution of alleles is inconsi stent with the dendrogram of the bacterial strains derived from other criteria. This discrepancy and the dissimilar coding sequences of alle lic hsd genes both imply lateral transfer of hsd genes.