KAS III-like enzymes play a significant role in natural product biosynthesis through C–C, C–O, and/or C–N bond formation.

Streptomyces rochei 7434AN4 produces two structurally unrelated polyketide antibiotics, lankacidin and lankamycin, and carries three linear plasmids, pSLA2-L (211 kb), -M (113 kb), and -S (18 kb), whose nucleotide sequences were previously reported. The complete nucleotide sequence of the S. rochei chromosome has now been determined using the long-read PacBio RS-II sequencing together with short-read Illumina Genome Analyzer IIx sequencing and Roche 454 pyrosequencing techniques. The assembled sequence revealed an 8,364,802-bp linear chromosome with a high G + C content of 71.7% and 7,568 protein-coding ORFs. Thus, the gross genome size of S. rochei 7434AN4 was confirmed to be 8,706,406 bp including the three linear plasmids. Consistent with our previous study, a tap-tpg gene pair, which is essential for the maintenance of a linear topology of Streptomyces genomes, was not found on the chromosome. Remarkably, the S. rochei chromosome contains seven ribosomal RNA ( rrn ) operons (16S-23S-5S), although Streptomyces species generally contain six rrn operons. Based on 2ndFind and antiSMASH platforms, the S. rochei chromosome harbors at least 35 secondary metabolite biosynthetic gene clusters, including those for the 28-membered polyene macrolide pentamycin and the azoxyalkene compound KA57-A.

Streptomyces rochei 7434AN4 carries three linear plasmids, pSLA2-L (211 kb), pSLA2-M (113 kb) and pSLA2-S (18 kb), their complete nucleotide sequences having been determined. Restriction and sequencing analysis revealed that the telomere sequences at both ends of the linear chromosome are identical to each other, are 98.5% identical to the right end sequences of pSLA2-L and pSLA2-M up to 3.1 kb from the ends and have homology to those of typical Streptomyces species. Mutant 2-39, which lost all the three linear plasmids, was found to carry a circularized chromosome. Sequence comparison of the fusion junction and both deletion ends revealed that chromosomal circularization occurred by terminal deletions followed by nonhomologous recombination. Curing of pSLA2-L from strain 51252, which carries only pSLA2-L, also resulted in terminal deletions in newly obtained mutants. The tap-tpg gene pair, which encodes a telomere-associated protein and a terminal protein for end patching, is located on pSLA2-L and pSLA2-M but has not hitherto been found on the chromosome. These results led us to the idea that the tap-tpg of pSLA2-L or pSLA2-M functions to maintain a linear chromosome in strain 7434AN4. This hypothesis was finally confirmed by complementation and curing experiments of the tap-tpg of pSLA2-M.

Lankacidin group antibiotics show strong antimicrobial activity against various Gram-positive bacteria. In addition, they were shown to have considerable antitumor activity against certain cell line models. For decades, the antitumor activity of lankacidin was associated with the mechanism of its antimicrobial action, which is interference with peptide bond formation during protein synthesis. This, however, was never confirmed experimentally. Due to significant similarity to paclitaxel-like hits in a previous computational virtual screening study, we suggested that the cytotoxic effect of lankacidin is due to a paclitaxel-like action. In this study, we tested this hypothesis computationally and experimentally and confirmed that lankacidin is a microtubule stabilizer that enhances tubulin assembly and displaces taxoids from their binding site. This study serves as a starting point for optimization of lankacidin derivatives for better antitumor activities. It also highlights the power of computational predictions and their aid in guiding experiments and formulating rigorous hypotheses.

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