Clethramycin (1) and mediomycin A (2) belong to the linear polyene polyketide (LPP) family of antibiotics that exhibit potent antifungal activity. Structural similarities exist between 1 and 2, except that 2 contains an amino moiety substituted for the guanidino moiety. Herein, the draft genome sequence of Streptomyces mediocidicus ATCC23936, a strain which produces both 1 and 2, was obtained through de novo sequencing. Bioinformatic analysis of the genome revealed a clethramycin (cle) gene cluster that contained 25 open reading frames (orfs). However, amidinohydrolase for 2 formation was not found in the cle gene cluster. Further genomic analysis revealed an amidinohydrolase MedX, which can hydrolyse the guanidino form (1) into the amino form (2) via heterologous co-expression of the cle cluster in Streptomyces lividans or by in vitro catalysis. These results also suggest the feasibility of engineering novel LPPs for drug discovery by manipulating the biosynthetic machinery of S. mediocidicus.

Streptomyces actuosus ATCC 25421 was famous for producing thiopeptide nosiheptide, which has widely been used as a feed additive for the promotion of animal growth. Herein, we report the complete genome sequence of S. actuosus ATCC 25421, which consists of an 8,145,579‐bp circular chromosome with a G+C content of 72.53 % containing 7 536 protein‐coding genes. The antiSMASH 3.0 program was used to identify 49 biosynthetic gene clusters for putative secondary metabolites, including a putative lantipeptide gene cluster that showed 85 % similarity to the reported informatipeptin biosynthetic gene cluster, indicating that the putative lantipeptide gene cluster has the ability to generate the informatipeptin analogue. Compared with avermipeptin, the lantipeptide precursor peptide (termed avermipeptin B) from S. actuosus ATCC 25421 contains a 14‐aa leader peptide and a 24‐aa core peptide, in which Ile15 was different from Val15 in avermipeptin. We also deduced the structure and the biosynthetic mechanism of avermipeptin B. Heterologous expression of the avermipeptin B biosynthetic gene cluster in S. lividans TK24 was characterized by high‐resolution mass spectrometry (ESI‐MS/MS). Finally, we found that avermipeptin B displayed strong activity against Gram‐positive strains. The genome sequence reported here can encourage us to mine novel secondary metabolites and investigate their biosynthetic mechanism in the future.

With the developments in taxonomy, the classically used highly conserved 16S rRNA molecular marker has shown some disadvantages among closely related species. For further taxonomic studies of the prokaryotes, specific PCR primers were designed from two conserved regions in the amino acid sequences of the 70-kDa heat shock protein sourced from 20 different genera in actinomycetes. These were used for the amplification of the hsp70 genes in 16 Streptomyces strains. Then, we investigated the phylogenetic relationships among these Streptomyces strains and compared the tree topology based on the hsp70 gene with those based on the previously used markers (16S rRNA and gyrB). To our knowledge, this is the first use of the hsp70 gene as a molecular marker for the taxonomic identification of Streptomyces.