Genomics-based approach towards the discovery of a lipopeptide from Herpetosiphon aurantiacus
Microbes provide a rich source for compounds of therapeutic importance. These metabolites are synthesized by enzymes encoded by associated biosynthetic gene clusters. Genome sequencing data from microbes revealed the presence of many gene clusters, for which the secondary metabolites are not known. These gene clusters are often referred to as orphan gene clusters (1) whereby the structure of the metabolites originating from this genetic information can be predicted by alignments with conserved regions of biosynthetic genes of known metabolites. Recently the genome of Herpetosiphon aurantiacus, a gram-negative, gliding bacterium (2),(3) was sequenced. From its genomic sequence, a putative PKS-NRPS (polyketide synthase-non-ribosomal peptide synthetase) orphan gene cluster involved in the biosynthesis of a lipopeptide has been identified. Bioinformatic analysis of this PKS-NRPS cluster revealed that the lipopeptide is composed of six amino acids, including the unusual amino acid p-hydroxy phenylglycine, and a carboxylic acid moiety. One of the NRPS adenylation domains was heterologously expressed and first experiments showed that p-hydroxy phenylglycine is activated. Expression analysis at different growth conditions showed that the PKS-NRPS genes are expressing in casitone-yeast based medium. Furthermore, the bacterium was cultivated in casitone-yeast based medium and extracted. The extract showed antibacterial activity and LCMS analysis revealed a compound with a molecular mass of 774, coinciding with that of the proposed lipopeptide. Bioactivity and LC-MS guided purification and structure elucidation of the compound is being carried on.
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