부산대학교 도서관

The occurrence of water blooms of cyanobacteria is a wild-wide problem in eutrophic lakes. In some cases, such blooms of cyanobacteria produce microcystins, which are known as the potent hepatotoxin and liver tumor promoter. Microcystins are cyclic heptapeptides and are non-ribosomally synthesized. For the regulation of the production of microcystins by toxic cyanobacteria, we have investigated their biosynthesis. Initially, we carried out the detection, isolation and structural determination of non-toxic peptides produced together with microcystins by toxic cyanobacteria. As shown in Table 1, the isolated non-toxic peptides were classified into several groups on the basis of their structures. The results suggested that the production of these non-toxic peptides is closely related to that of microcystins. In the course of our investigations, Shirai et al. recently identified whole microcystin synthetase genes in the toxic cyanobacterium, Microcystis aeruginosa K-139, which produces 3-desmethyl and 3,7-didesmethyl microcystin-LR (Fig. 1, 2(a)). Furthermore, the synthetic genes of a peptide other than microcystins were also isolated from the same strain (Fig. 2(b)). In this study, we carried out the isolation and structural determination of the corresponding peptide to the synthetic genes isolated. As the result, we isolated a non-toxic peptide designated as micropeptin K139 (1) together with two microcystins from M. aeruginosa K-139. Micropeptin K139 (1) is a cyclic depsipeptide composed of seven amino acids, L-Ile x2, L-MeTyr, L-Ahp, L-Arg, L-Thr, L-Asp, and a fatty acid, HA, and belongs to a group of non-toxic peptides (Fig. 1). The sequence of the constituent segments in 1 and its structure were determined by 2D-NMR techniques, and the advanced Marfey's method was used to determine their absolute configurations. The findings about the structure of micropeptin K139 (1) and the isolated peptide synthetase genes other than those of microcystins indicated that these synthetic genes are for a nontoxic peptide, 1. Therefore, the result suggested that non-toxic peptides are also non-ribosomally synthesized as well as microcystins. The investigation of the genetic relationship between microcystins and the co-producing peptides is in progress.