Mechanism of action
The antibacterial activity of rifamycins relies on the inhibition of bacterial DNA-dependent RNA synthesis. This is due to the high affinity of rifamycins for the  prokaryotic RNA polymerase. The selectivity of the rifamycins depends on the fact that they have a very poor affinity for  the analogous mammalian enzyme. Crystal structure data of the antibiotic bound to RNA polymerase indicates that rifamycin blocks synthesis by causing strong steric clashes with the growing oligonucleotide ("steric-occlusion" mechanism). If rifamycin binds the polymerase after the chain extension process has started, no inhibition is observed on the biosynthesis, consistent with a steric-occlusion mechanism. Single step high level resistance to the rifamycins occurs as the result of a single amino acid change in the bacterial DNA dependent RNA polymerase. Biosynthesis
The first information on the biosynthesis of the rifamycins came from studies using the stable isotope Carbon-13 and NMR spectroscopy to establish the origin of the carbon skeleton. These studies showed that the ansa chain was derived from acetate and propionate, in common with other polyketide antibiotics. The naphthalenic chromophore was shown to derive from a propionate unit coupled with a seven carbon amino moiety of unknown origin. The general scheme of biosynthesis starts with the uncommon starting unit, 3-amino-5-hydroxybenzoic acid (AHBA), via type I polyketide pathway (PKS I) in which chain extension is performed using 2 acetate and 8 propionate units. AHBA is believed to have originated from the Shikimate pathway, however this was not incorporated into the biosynthetic mechanism.
(1969), 67 147-151 - the paper which renamed Streptomyces mediterranei as Nocardia mediterranei. Lechevalier et al., Int. J. Syst. Bacteriol. (1986), 36, 29) - the paper which renamed Nocardia mediterranei as Amycolatopsis mediterranei. Bala S (2004). "Reclassification of "Amycolatopsis mediterranei" DSM 46095 as "Amycolatopsis rifamycinica" sp. nov". International Journal of Systematic and Evolutionary Microbiology. 54 (4): 1145–1149. doi:10.1099/ijs.0.02901-0. PMID 15280283. - the paper with the latest name change

External links
"Rifamycin". Drug Information Portal. U.S. National Library of Medicine.