AN AMPLIFIABLE AND DELETABLE LOCUS OF STREPTOMYCES-AMBOFACIENS RP181110 CONTAINS A VERY LARGE GENE HOMOLOGOUS TO POLYKETIDE SYNTHASE GENES

Streptomyces ambofaciens RP181110 produces the macrolide polyketide sp iramycin. Like many other Streptomyces species, the RP181110 strain is prone to genetic instability involving genomic rearrangements (deleti ons and/or amplifications) in the large unstable region of the genome. It has previously been demonstrated that the amplification of a parti cular locus (AUD205) affects spiramycin biosynthesis and, conversely, the loss of this amplification is correlated with the restoration of a ntibiotic production. This report focuses on a 0.93 kb reiterated frag ment specific for the AUD205 locus. Sequencing of 3596 bp including th is reiteration revealed the presence of an ORF (orfPS) whose potential product was highly homologous to the EryA and Raps proteins, responsi ble for the biosynthesis of erythromycin in Saccharopolyspora erythrae a and rapamycin in Streptomyces hygroscopicus, respectively. orfPS enc odes a protein with at least four successive domains: ketoacyl synthas e, acyltransferase, ketoreductase and acyl carrier protein. This organ ization is very similar to most eryA and rap modules. The reiterated s equence corresponds to the acyltransferase domain. orfPS was transcrib ed during rapid growth and stationary phase in RP181110 and overtransc ribed in the amplified mutant. Both these results suggest that the gen e encodes a type I polyketide synthase and its reorganization is respo nsible for the loss of spiramycin production in the amplified strains.