**Anna Fidor 1, Michał Grabski 2, Jan Gawor 3, Robert Gromadka 3, Grzegorz W˛egrzyn <sup>2</sup> and Hanna Mazur-Marzec 1,\***


Received: 31 July 2020; Accepted: 25 August 2020; Published: 26 August 2020

**Abstract:** Nostocyclopeptides (Ncps) constitute a small class of nonribosomal peptides, exclusively produced by cyanobacteria of the genus *Nostoc*. The peptides inhibit the organic anion transporters, OATP1B3 and OATP1B1, and prevent the transport of the toxic microcystins and nodularin into hepatocytes. So far, only three structural analogues, Ncp-A1, Ncp-A2 and Ncp-M1, and their linear forms were identified in *Nostoc* strains as naturally produced cyanometabolites. In the current work, the whole genome sequence of the new Ncps producer, *N*. *edaphicum* CCNP1411 from the Baltic Sea, has been determined. The genome consists of the circular chromosome (7,733,505 bps) and five circular plasmids (from 44.5 kb to 264.8 kb). The nostocyclopeptide biosynthetic gene cluster (located between positions 7,609,981–7,643,289 bps of the chromosome) has been identified and characterized *in silico*. The LC-MS/MS analyzes of *N*. *edaphicum* CCNP1411 cell extracts prepared in aqueous methanol revealed several products of the genes. Besides the known peptides, Ncp-A1 and Ncp-A2, six other compounds putatively characterized as new noctocyclopeptide analogues were detected. This includes Ncp-E1 and E2 and their linear forms (Ncp-E1-L and E2-L), a cyclic Ncp-E3 and a linear Ncp-E4-L. Regardless of the extraction conditions, the cell contents of the linear nostocyclopeptides were found to be higher than the cyclic ones, suggesting a slow rate of the macrocyclization process.

**Keywords:** cyanobacteria; nostocyclopeptides; *Nostoc*; *ncp* gene cluster; nonribosomal peptide synthetase
