Biochemical study of R-75-1, a new semi-synthetic rifamycin.

A new derivative of rifamycin SV, R-75-1, inhibits RNA synthesis in Mycobacterium smegmatis ATCC 607 (M607) at a concentration 10 times lower than rifampicin (RFP). However, both R-75-1 and RFP inhibit RNA polymerase reaction by 50% at the same concentration level (0.05 approximately 0.1 microgram/ml). Both inhibit the initiation process of RNA synthesis. E. coli RNA polymerase of the RFP-resistant strain was resistant to R-75-1. RFP was not inactivated by M607 cell extracts. The inhibitory effect of R-75-1 is markedly diminished if mycobacteria are grown in the medium containing Tween 80. On the basis of these results, the greater activity of R-75-1 to mycobacteria is suggested to be due to the better permeability than RFP.