Genetic interrelations in the actinomycin biosynthetic gene clusters of <em>Streptomyces antibioticus</em> IMRU 3720 and <em>Streptomyces chrysomallus</em> ATCC11523, producers of actinomycin X and actinomycin C

Abstract

Sequencing the actinomycin (acm) biosynthetic gene cluster of Streptomyces antibioticus IMRU 3720, which produces actinomycin X (Acm X), revealed 20 genes organized into a highly similar framework as in the bi-armed acm C biosynthetic gene cluster of Streptomyces chrysomallus but without an attached additional extra arm of orthologues as in the latter. Curiously, the extra arm of the S. chrysomallus gene cluster turned out to perfectly match the single arm of the S. antibioticus gene cluster in the same order of orthologues including the the presence of two pseudogenes, scacmM and scacmN, encoding a cytochrome P450 and its ferredoxin, respectively. Orthologues of the latter genes were both missing in the principal arm of the S. chrysomallus acm C gene cluster. All orthologues of the extra arm showed a G +C-contents different from that of their counterparts in the principal arm. Moreover, the similarities of translation products from the extra arm were all higher to the corresponding translation products of orthologue genes from the S. antibioticus acm X gene cluster than to those encoded by the principal arm of their own gene cluster. This suggests that the duplicated structure of the S. chrysomallus acm C biosynthetic gene cluster evolved from previous fusion between two one-armed acm gene clusters each from a different genetic background. However, while scacmM and scacmN in the extra arm of the S. chrysomallus acm C gene cluster are mutated and therefore are non-functional, their orthologues saacmM and saacmN in the S. antibioticus acm C gene cluster show no defects seemingly encoding active enzymes with functions specific for Acm X biosynthesis. Both acm biosynthetic gene clusters lack a kynurenine-3-monooxygenase gene necessary for biosynthesis of 3-hydroxy-4-methylanthranilic acid, the building block of the Acm chromophore, which suggests participation of a genome-encoded relevant monooxygenase during Acm biosynthesis in both S. chrysomallus and S. antibioticus.

Ketonization of Proline Residues in the Peptide Chains of Actinomycins by a 4‐Oxoproline Synthase

“…Previous genomic analysis of S. antibioticus and S. chrysomallus revealed their Acm biosynthetic gene clusters . The S. antibioticus Acm X biosynthetic gene cluster contains two genes, sa acmM and sa acmN , encoding a cytochrome P450 of 434 amino‐acid residues and its ferredoxin (68 residues), respectively (Scheme ).…”

Comparison of actinomycin peptide synthetase formation in Streptomyces chrysomallus and Streptomyces antibioticus

“…Actinomycin C is a rare family of actinomycins distinct from actinomycins X and D. They preferentially contain D‐ allo ‐isoleucine in the 2‐positions of their peptide lactone rings as result of a different substrate specificity‐determining residues in the A‐domain of the second module in AcmB . Moreover, the actinomycin C biosynthetic gene cluster of S. chrysomallus does not contain the genes encoding the post‐assembly modification of prolines such as in the actinomycin X biosynthetic gene cluster of S. antibioticus . Actinomycin G, Y, and Z are characterized by the presence of additional rings in the phenoxazinone ring system and a number of special modifications of imino acids in the 3‐positions of their pentapetide lactone rings .…”

“…Previous work in our laboratory unveiled the structure of the actinomycin biosynthetic gene clusters of S. chrysomallus and of S. antibioticus . Both gene clusters are highly similar in their gene arrangements and gene sequences.…”

“…From this, no role of these repeats in regulation of synthetase formation could be attributed. The absence of pathway‐specific regulatory genes in the acm biosynthetic gene clusters therefore points to control of actinomycin formation by pleiotropic regulators, which—depending on their absence or presence in different strains—could control actinomycin production in a species‐specific manner.…”

“…However, no promoters of genes involved in actinomycin biosynthesis have been analyzed yet. Recently, we sequenced the genome of S. antibioticus and could compare its actinomycin biosynthetic gene cluster with that from S. chrysomallus . Both gene clusters are highly homologous and possess the same inverted arrangement of peptide synthetase genes.…”