The biologically active compound mensacarcin is produced by Streptomyces bottropensis. The cosmid cos2 contains a large part of the mensacarcin biosynthesis gene cluster. Heterologous expression of this cosmid in Streptomyces albus J1074 led to the production of the intermediate didesmethylmensacarcin (DDMM). In order to gain more insights into the biosynthesis, gene inactivation experiments were carried out by λ-Red/ET-mediated recombination, and the deletion mutants were introduced into the host S. albus. In total, 23 genes were inactivated. Analysis of the metabolic profiles of the mutant strains showed the complete collapse of DDMM biosynthesis, but upon overexpression of the SARP regulatory gene msnR1 in each mutant new intermediates were detected. The compounds were isolated, and their structures were elucidated. Based on the results the specific functions of several enzymes were determined, and a pathway for mensacarcin biosynthesis is proposed.

The biological active compound rishirilide B is produced by Streptomyces bottropensis. The cosmid cos4 contains the complete rishirilide B biosynthesis gene cluster. Its heterologous expression in the host Streptomyces albus J1074 led to the production of rishirilide B as a major compound and to small amounts of rishirilide A, rishirilide D and lupinacidin A. In order to gain more insights into the biosynthesis, gene inactivation experiments and gene expression experiments were carried out. This study lays the focus on the functional elucidation of the genes involved in the early biosynthetic pathway. A total of eight genes were deleted and six gene cassettes were generated. Rishirilide production was not strongly affected by mutations in rslO2, rslO6 and rslH. The deletion of rslK4 and rslO3 led to the formation of polyketides with novel structures. These results indicated that RslK4 and RslO3 are involved in the generation or selection of the starter unit for rishirilide biosynthesis. In the rslO10 mutant strain, two novel compounds were detected, which were also produced by a strain containing solely the genes rslK1, rslK2, rslK3, rslK4, and rslA. rslO1 and rslO4 mutants predominately produce galvaquinones. Therefore, the ketoreductase RslO10 is involved in an early step of rishirilide biosynthesis and the oxygenases RslO1 and RslO4 are most probably acting on an anthracene moiety. This study led to the functional elucidation of several genes of the rishirilide pathway, including rslK4, which is involved in selecting the unusual starter unit for polyketide synthesis.Molecules 2020, 25, 1955 2 of 15 the initiation module, can be found, consisting of a ketosynthase, a malonyl CoA:ACP transferase and an acyl carrier protein (ACP). The initiation module selects the starter unit and catalyzes the first elongation of the growing polyketide, before the molecule is loaded onto the minPKS for further elongation reactions. Additional enzymes, like ketoreductases, cyclases and aromatases complement the minPKS and determine the folding of the respective polyketide [1]. The aromatic polyketide rishirilide B (Figure 1) was first discovered by Iwaki et al. in 1984. Rishirilide B was isolated from Streptomyces rishiriensis OFR-1056 in the course of screening for new α2-macroglobulin inhibitors [2]. This compound was also described as a product of S. olivaceus SCSIO T05, which also produced rishirilide C, lupinacidin A and galvaquinone A and B [3] (Figure 1). The biosynthetic gene cluster of this strain responsible for the formation of all four compounds was cloned and biosynthetic studies were performed [3]. S. bottropensis (formerly S. sp. Gö C4/4) also produces the compounds rishirilide A, B, D and lupinacidin A (Figure 1). The corresponding rishirilide biosynthetic gene cluster was cloned on a cosmid (cos4), which was subsequently expressed in the heterologous host S. albus J1074. Rishirilide B was produced as a major compound. Moreover, rishirilide A, D and lupinacidin A were also produced in small amounts [3] (Figure 1)....

Recently we described an unusual way of activating a cryptic gene cluster when we explored the origin of the bald phenotype of Streptomyces calvus. Complementation of S. calvus with a correct copy of bldA restored sporulation and additionally promoted production of a new natural products. In this study we report on the expression of bldA in several Streptomyces strains that have been described as "poorly sporulating" strains. In seven out of 15 cases, HPLC profiling revealed the production of new compounds, and in two cases the overproduction of known compounds. Two compounds were isolated and their structures were determined.

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