The StrQ protein encoded in the gene cluster for 5′‐hydroxystreptomycin of Streptomyces glaucescens GLA.0 is a α‐D‐glucose‐1‐phosphate cytidylyltransferase (CDP‐D‐glucose synthase)

The gene strQ was identified as the last gene of a putative transcription unit, strB1FGHPQ, located in the gene cluster for the production of 5′-hydroxy-streptomycin (OH-Sm) in Streptomyces glaucescens GLA.0. [In contrast, the corresponding operon in the str/sts-gene cluster of the Sm-producer Streptomyces griseus, strB1FGHIK, differs in the two distal genes; Mansouri, K. & Piepersberg, W. (1991) Mol. Gen. Genet. 228, 459−469.] The deduced StrQ protein exhibited similarities to members of the enzyme family of hexose-1-phosphate nucleotidylyltransferases (NDP-hexose synthases or pyrophosphorylases), with the strongest similarity to the subfamily of α- D-glucose-1-phosphate cytidylyltransferases (CDP- D-glucose synthases). The StrQ protein was heterologously expressed in Escherichia coli. The purified protein revealed an enzyme activity of that of a CDP- D-glucose synthase and a substrate specificity restricted to CTP and α- D-glucose 1-phosphate. The Km and Vmax values determined for CTP are 44 μM and 920 μM and for α- D-glucose 1-phosphate 195 μM and 1.06 mM, respectively. The CDP- D-glucose synthase activity was also detected in cells of S. glaucescens under the conditions of antibiotic production, but was absent from cells of the streptomycin producer S. griseus N2-3-11. Also, the genomes of several strains of S. griseus did not seem to possess strQ-related genes. In contrast, hybridisation experiments indicated that genes homologous to strQ were probably present in various other actinomycetes producing aminoglycosides. A possible function of the StrQ protein in the OH-Sm biosynthetic pathway of GLA.0 is discussed.