The Biosynthesis of Kasugamycin, an Antibiotic against Rice Blast Disease, with Particular Reference to the Involvement of rpoZ, a Gene Encoding RNA Polymerase Omega Subunit

Biosynthetic genes of kasugamycin (KSM), an aminoglycoside antibiotic produced by Streptomyces kasugaensis, forms a cluster within the chromosome. A cloned 22.4-kb cluster region contains almost all the enzyme-coding genes required for KSM biosynthesis together with kac338, a gene for KSM acetyltransferase, and kasKLM, a set of genes encoding an ABC transporter, both of which participate in KSM self-resistance as well as kasT encoding a KSM-synthesis-specific transcriptional activator of the biosynthetic genes. Furthermore, rpoZ, encoding a 90-amino acid omega (ω) subunit of RNA polymerase (RNAP), is required for the simultaneous production of KSM and aerial mycelium, and rpoZ-disrupted wild-type S. kasugaensis produces neither KSM nor aerial mycelia. Transcriptional analysis of the biosynthetic genes and forcible expression of kasT in the mutant revealed that the presence of rpoZ, which results in formation of RNAP carrying the ω subunit, facilitates initiation of kasT transcription and is thus crucial for KSM production.