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)

Abstract

<jats:p>The gene <jats:italic>strQ</jats:italic> was identified as the last gene of a putative transcription unit, <jats:italic>strB1FGHPQ</jats:italic>, located in the gene cluster for the production of 5′‐hydroxy‐streptomycin (OH‐Sm) in <jats:italic>Streptomyces glaucescens</jats:italic> GLA.0. [In contrast, the corresponding operon in the <jats:italic>str</jats:italic>/<jats:italic>sts‐</jats:italic>gene cluster of the Sm‐producer <jats:italic>Streptomyces griseus</jats:italic>, <jats:italic>strB1FGHIK</jats:italic>, differs in the two distal genes; Mansouri, K. &amp; Piepersberg, W. (1991) <jats:italic>Mol. Gen. Genet. 228</jats:italic>, 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‐ <jats:sc>D</jats:sc>‐glucose<jats:italic>‐</jats:italic>1‐phosphate cytidylyltransferases (CDP‐ <jats:sc>D</jats:sc>‐glucose synthases). The StrQ protein was heterologously expressed in <jats:italic>Escherichia coli</jats:italic>. The purified protein revealed an enzyme activity of that of a CDP‐ <jats:sc>D</jats:sc>‐glucose synthase and a substrate specificity restricted to CTP and α‐ <jats:sc>D</jats:sc>‐glucose 1‐phosphate. The <jats:italic>K</jats:italic><jats:sub>m</jats:sub> and <jats:italic>V</jats:italic><jats:sub>max</jats:sub> values determined for CTP are 44 μM and 920 μM and for α‐ <jats:sc>D</jats:sc>‐glucose 1‐phosphate 195 μM and 1.06 mM, respectively. The CDP‐ <jats:sc>D</jats:sc>‐glucose synthase activity was also detected in cells of <jats:italic>S. glaucescens</jats:italic> under the conditions of antibiotic production, but was absent from cells of the streptomycin producer <jats:italic>S. griseus</jats:italic> N2‐3‐11. Also, the genomes of several strains of <jats:italic>S. griseus</jats:italic> did not seem to possess <jats:italic>strQ</jats:italic>‐related genes. In contrast, hybridisation experiments indicated that genes homologous to <jats:italic>strQ</jats:italic> 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.