Rifamicin and its
synthetic derivative the
antibiotic rifampicin, derived from
Streptomyces strains, specifically
inhibit initiation of
RNA synthesis in
prokaryotes. They don't do that by blocking the binding of
RNA polymerase, but it screws up the the formation of the first
phosphodiester bond in the RNA chain. As with a lot of other antibiotics, they don't affect anything else except this target. This feature make the substance a cool
research tool: it can be used to block the initiation of new RNA chains without interfering with the ones already in progress.
The site of action is the beta-subunit of RNA
polymerase. And guess what? Some bacteria have an altered beta chain which make them
resistant.