Thioesterases and the premature termination of polyketide chain elongationin rifamycin B biosynthesis by Amycolatopsis mediterranei S699

The role of two thioesterase genes in the premature release of polyketide s ynthase intermediates during rifamycin biosynthesis in the Amycolatopsis me diterranei S699 strain was investigated. Creation of:an in-frame deletion i n the rifR gene led to a 30 similar to 60% decrease in the production of bo th rifamycin B by the S699 strain or a series of tetra- to decaketide shunt products of polyketide chain assembly by the rifF strain. Since a similar percentage decrease was seen in both genetic backgrounds, we conclude that the RifR thioesterase 2 is not involved in premature release of the carbon chain assembly intermediates. Similarly, fusion of the Saccharopolyspora er ythraea DEBS3 thioesterase 1 domain to the C-terminus of the RifE PKS subun it did not result in a noticeable increase in the amount of the undecaketid e intermediate formed nor in the amounts of the tetra- to decaketide shunt products. Hence, premature release of the carbon chain assembly intermediat es is an unusual property of the Rif PKS itself.