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.