Purification and Biochemical Characterization of Mutacin I from the Group I Strain of Streptococcus mutans , CH43, and Genetic Analysis of Mutacin I Biosynthesis Genes

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Abstract
Previously, we reported isolation and characterization of mutacin III and genetic analysis of mutacin III biosynthesis genes from the group III strain of Streptococcus mutans , UA787 (F. Qi, P. Chen, and P. W. Caufield, Appl. Environ. Microbiol. 65:3880–3887, 1999). During the same process of isolating the mutacin III structural gene, we also cloned the structural gene for mutacin I. In this report, we present purification and biochemical characterization of mutacin I from the group I strain CH43 and compare mutacin I and mutacin III biosynthesis genes. The mutacin I biosynthesis gene locus consists of 14 genes in the order mutR , - A , - A ′, - B , - C , - D , - P , - T , - F , - E , - G , orfX , orfY , orfZ. mutA is the structural gene for mutacin I, while mutA ′ is not required for mutacin I activity. DNA and protein sequence analysis revealed that mutacins I and III are homologous to each other, possibly arising from a common ancestor. The mature mutacin I is 24 amino acids in size and has a molecular mass of 2,364 Da. Ethanethiol modification and peptide sequencing of mutacin I revealed that it contains six dehydrated serines, four of which are probably involved with thioether bridge formation. Comparison of the primary sequence of mutacin I with that of mutacin III and epidermin suggests that mutacin I likely has the same bridging pattern as epidermin.