Distribution and evolution of chitinase genes in Streptomyces species: involvement of gene-duplication and domain-deletion.

Streptomyces coelicolor A3(2) possesses nine genes for family 18 chitinases and two for family 19, showing high multiplicity. By hybridization analyses, distribution of those chitinase genes was investigated in six other Streptomyces species covering the whole phylogenetic range based on 16S rDNA sequences. All strains showed high-multiplicity of chitinase genes, like S. coelicolor A3(2). The phylogeny and gene organization of the family 18 chitinase genes cloned from Streptomyces species so far were then analyzed to investigate the gene evolution. It was concluded that Streptomyces already possessed a variety of chitinase genes prior to branching into many species, and that the ancestral genes of chiA and chiB have been generated by gene-duplication. In the course of the analyses, evidence that the chi30 and chi40 genes of S. thermoviolaceus were derived from their corresponding original chitinase genes by losing gene parts for substrate-binding domains and fibronectin type III-like domains was obtained. It was thus shown that gene-duplication and domain-deletion were implicated in generating the high diversity and multiplicity of chitinase genes in Streptomyces species.