Chromosomal Deletions in Melibiose-negative Isolates of Streptococcus mutans

Abstract

Isolates from a collection of phenotypically melibiose-negative (Mel-) Streptococcus mutans from widely-scattered geographical locations were examined and found to lack the activities of the enzymes α-galactosidase and a-glucosidase, in addition to being unable to transport melibiose. Cloned fragments of S. mutans DNA from the region of the chromosome carrying the genes for a-galactosidase ( aga), sucrose phosphorylase ( gtfA), and dextranglucosidase ( dexB), as well as the genes encoding components of the binding-protein-dependent uptake system for raffinose and melibiose, were used in hybridization studies for investigation of the genetic basis of the Mel- phenotype. A region of at least 12 kilobases, containing all the above genes, was found to be deleted from the chromosome of the Mel- strains. It appears that this region of the chromosome is not essential for survival of S. mutans in the oral cavity. The reason for the frequent occurrence of deletions, as opposed to other forms of mutational events, is unknown.