The dasABC Gene Cluster, Adjacent to dasR, Encodes a Novel ABC Transporter for the Uptake of N,N′-Diacetylchitobiose in Streptomyces coelicolor A3(2)

Streptomycetes are known as saprophytic soil bacteria and the main decomposers of chitin, which is a polymer of N-acetylglucosamine (GlcNAc) linked by β-1,4 bonds. Streptomycetes degrade chitin by producing a variety of chitinase (EC 3.2.1.14) (for a review, see reference 27) that hydrolyzes chitin into N,N′-diacetylchitobiose [(GlcNAc)2] as the final product (3, 21). Streptomyces coelicolor A3(2) strain M145 possesses 13 chitinase (chi) genes in its genome, although 6 of them are putative ones (2, 6, 12, 29). The expression of five chi genes (chiA, chiB, chiC, chiD, and chiF) is induced by chitin in the bacterium (30), and each of the chitinases (ChiA, ChiC, ChiD, and ChiF) has its own enzymatic properties (12). It is assumed that these chitinases act synergistically with each other in chitin degradation, as has been reported in the chitinase system of a marine bacterium, Alteromonas sp. strain O-7 (22). It is generally known that chitinase genes in streptomycetes are induced in the presence of the chitin substrate and repressed when both glucose (Glc) and chitin are present (for a review, see reference 27). The regulation mechanisms are, however, still unclear. The 12-bp direct repeat (DR) sequences overlapping the promoters are required for regulation of the chi63 gene in Streptomyces plicatus and the chiA gene in Streptomyces lividans (8, 16, 20). The DR-like sequences are also present upstream of other chitinase genes, the expression of which is regulated as described above. It is therefore believed that the DR-like sequences are common cis-acting elements involved in the regulation of chitinase genes in streptomycetes. The transcription of chi genes in S. coelicolor A3(2) and S. lividans is induced by (GlcNAc)2 as well as by chitin (17, 30). Because of the more immediate induction activity of (GlcNAc)2, it has been suggested that (GlcNAc)2 production, which occurs by hydrolysis of chitin, is necessary for the induction of chi gene expression (30). Although chitinase systems have been studied in streptomycetes for decades, the nature of the systems of transport of chitin degradation products was unclear until two transporters for GlcNAc uptake were identified in Streptomyces olivaceoviridis. One is Ngc, an ABC (ATP-binding cassette) transport system for GlcNAc and probably for (GlcNAc)2 (40). The corresponding solute-binding protein NgcE has high affinities for GlcNAc and (GlcNAc)2. The other transporter protein is PtsC2 of the phosphotransferase system which is specific for GlcNAc (38). During the course of our studies, we obtained data suggesting that (GlcNAc)2 uptake activity was induced by the amino sugar itself. In light of this implication, we explored transporter genes for (GlcNAc)2 in the genome of S. coelicolor A3(2) by using the DR sequence as a probe. We successfully identified a novel ABC transporter gene for (GlcNAc)2, the deduced products of which exhibit properties distinct from those of Ngc of S. olicaceoviridis. Interestingly, the gene cluster for the new transporter is located adjacent to the dasR gene, which is involved in the regulation of morphological differentiation in Streptomyces griseus (35) as well as in S. coelicolor A3(2) (23).