Identification and analysis of the chivosazol biosynthetic gene cluster from the myxobacterial model strain Sorangium cellulosum So ce56.

Myxobacteria belonging to the genus Sorangium are known to produce a variety of biologically active secondary metabolites. Chivosazol is a macrocyclic antibiotic active against yeast, filamentous fungi and especially against mammalian cells. The compound specifically destroys the actin skeleton of eucaryotic cells and does not show activity against bacteria. Chivosazol contains an oxazole ring and a glycosidically bound 6-deoxyglucose (except for chivosazol F). In this paper we describe the biosynthetic gene cluster that directs chivosazol biosynthesis in the model strain Sorangium cellulosum So ce56. This biosynthetic gene cluster spans 92 kbp on the chromosome and contains four polyketide synthase genes and one hybrid polyketide synthase/nonribosomal peptide synthetase gene. An additional gene encoding a protein with similarity to different methyltransferases and presumably involved in post-polyketide modification was identified downstream of the core biosynthetic gene cluster. The chivosazol biosynthetic gene locus belongs to the recently identified and rapidly growing class of trans-acyltransferase polyketide synthases, which do not contain acyltransferase domains integrated into the multimodular megasynthetases.

Characterisation, genome size and genetic manipulation of the myxobacterium Sorangium cellulosum So ce56.

In this study, Sorangium cellulosum So ce56 was phenotypically and genotypically analysed in order to evaluate whether this strain can be used in a comprehensive genome project as a representative of the secondary metabolite-producing myxobacteria. In contrast to many other strains of S. cellulosum, strain So ce56 was found to have various advantageous features, including fast and homogeneous growth in submerged cultures and the ability to complete its morphological differentiation cycle on agar, even when the inoculant originates from a liquid culture. Two groups of secondary metabolites isolated from the culture broth were identified, the polyketides etnangien and chivosazole. The presence of polyketide synthase-encoding genes in the genome of strain So ce56 was demonstrated via PCR. The phenotypic classification was confirmed by comparison of 16S rDNA sequences which showed that S. cellulosum So ce56 clusters within a separate lineage together with S. cellulosum ATCC 25531 and the epothilone producer S. cellulosum So ce90. The genome of S. cellulosum So ce56 belongs to the largest bacterial genomes described so far. It is estimated to be 12.2 Mb in size, by pulsed-field gel electrophoresis. In order to demonstrate that S. cellulosum So ce56 is a convenient strain for molecular biological studies, a genetic manipulation system was developed. Using triparental mating, polyketide synthase-encoding genes were inactivated, leading to chivosazole-negative mutants.