SanG, a transcriptional activator, controls nikkomycin biosynthesis through binding to the sanN–sanO intergenic region in Streptomyces ansochromogenes

Streptomyces ansochromogenes SanG is a pathway-specific regulator that mainly controls the transcription of two transcriptional units involved in nikkomycin biosynthesis. SanG consists of three major functional domains: an N-terminal Streptomyces antibiotic regulatory protein (SARP) domain, a central ATPase domain, and a C-terminal half homologous to guanylate cyclases belonging to the LuxR family. SanG was expressed in Escherichia coli as a C-terminally His(6)-tagged protein. The purified SanG-His( 6) was shown to be a dimer in solution by dynamic light scattering. An electrophoretic mobility-shift assay showed that the purified SanG protein could bind to the DNA fragment containing the bidirectional sanN-sanO promoter region. The SanG-binding sites within the bidirectional sanN-sanO promoter region were determined by footprinting analysis and identified a consensus-directed repeat sequence 5'-CGGCAAG-3'. SanG showed significant ATPase/GTPase activity in vitro, and addition of ATP/GTP enhanced the affinity of SanG for target DNA, but ATP/GTP hydrolysis was not essential for SanG binding to the target DNA. However, real-time reverse transcription PCR showed that mutation of the ATPase/GTPase domain of SanG significantly decreased the transcriptional level of sanN-I and sanO-V. These results indicated that the ATPase/GTPase activity of SanG modulated the transcriptional activation of SanG target genes during nikkomycin biosynthesis.