Molecular cloning and characterization of scrB, the structural gene for the Streptococcus mutans phosphoenolpyruvate-dependent sucrose phosphotransferase system sucrose-6-phosphate hydrolase

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Abstract
A DNA fragment encoding the sucrose-6-phosphate hydrolase component of the Streptococcus mutans phosphoenolpyruvate-dependent sucrose phosphotransferase system has been recovered from a plasmid-based genomic library of strain GS5. The locus, designated scrB, was found to reside within a 2.9-kilobase-pair restriction fragment present on the chimeric molecule pVA1343 (7.3 kilobase pairs). Minicell analysis of pVA1343-directed translation products revealed that the scrB product synthesized in Escherichia coli V1343 was a single peptide of Mr 57,000. This polypeptide was reactive with antiserum prepared against S. mutans intracellular invertase, which has been previously shown to have an Mr of 43,000 to 48,000. The basis of this difference in Mr was not established but may represent a posttranslational proteolytic event which occurred in S. mutans but not in recombinant V1343. Sucrose-6-phosphate hydrolase purified to homogeneity from V1343 exhibited Michaelis constants of 180 mM for sucrose and 0.08 mM for sucrose-6-phosphate. Deletion analysis of pVA1343 facilitated the assignment of a coding region for the hydrolase within the insert, as well as an orientation for the transcription of scrB. scrB-defective strains of S. mutans constructed by additive integration of an insertionally inactivated scrB locus exhibited the sucrose sensitivity characteristic of this mutant class. Similar loci were detected by DNA-DNA hybridization in additional strains of S. mutans and two strains of Streptococcus cricetus, but not in single strain representatives of S. rattus, S. sobrinus, S. sanguis I and II, S. salivarius, or S. mitis.