**1. Introduction**

Secondary metabolites produced by cyanobacteria of the genus *Nostoc* (Nostocales) are characterized by a high variety of structures and biological activities [1–6]. On the basis of chemical structure, these compounds are mainly classified to peptides, polyketides, lipids, polysaccharides and alkaloids [7]. Abundantly produced cyanopeptides with anticancer, antimicrobial, antiviral and enzyme-inhibiting activity, have attracted attention of many research groups [6,8–12]. Some of the metabolites, such as nostocyclopeptides (Ncps) or cryptophycins are exclusively produced by the cyanobacteria of the genus *Nostoc* (Figure 1A). Ncps constitute a small class of nonribosomal peptides. Thus far, only three analogues of the compounds and their linear forms have been discovered. This includes Ncp-A1 and Ncp-A2 detected in *Nostoc* sp. ATCC53789 isolated from a lichen collected at Arron Island in Scotland [13]. The same peptides were detected in *Nostoc* sp. ASN\_M, isolated from soil samples of paddy fields in the Golestan province in Iran [14] and in *Nostoc* strains from

liverwort *Blasia pusilla* L. collected in Northern Norway [15]. A different analogue, Ncp-M1, was found in *Nostoc* sp. XSPORK 13A, the cyanobacterium living in symbiosis with gastropod from shallow seawaters at the Cape of Porkkala (Baltic Sea) [16].

**Figure 1.** *Nostoc edaphicum* CCNP1411 (**A**) and the proposed chemical structure of Ncp-E1 (**B**).

Ncps are composed of seven residues and a unique imino linkage formed between C-terminal aldehyde and an *N*-terminal amine group of the conserved Tyr<sup>1</sup> (Figure 1B) [13,16]. The presence of modified amino acid residues, e.g., 4-methylproline, homoserine and D-configured glutamine, indicated the nonribosomal biosynthetic pathway of the molecules. Genetic analysis of *Nostoc* sp. ATCC53789 revealed the presence of the 33-kb Ncp gene cluster composed of two genes, *ncpA* and *ncpB*, encoding NcpA1-A3 and NcpB1-B4 modules. These proteins catalyze the activation and incorporation of Tyr, Gly, Gln, Ile and Ser into the Ncp structure [17]. They show high similarity to NosE and NosF which take part in the biosynthesis of nostopeptolides in *Nostoc* sp. GSV224 [18]. The *ncpFGCDE* fragment of the Ncp gene cluster is involved in the synthesis of MePro (*ncpCDE*), transport (*ncpF*) and proteolysis (*ncpG*) of the peptides. The characteristic features of the Ncp enzymatic complex in *Nostoc* sp. ATCC53789 is the presence of the epimerase domain (NcpA3) responsible for D-configuration of glutamine, and the unique reductase domain at C-terminal end of NcpB which catalyze the reductive release of a linear peptide aldehyde [19,20].

The activity of Ncps have been explored [13] and their potential as antitoxins, inhibiting the transport of hepatotoxic microcystin-LR and nodularin into the rat hepatocytes through the organic anion transporter polypeptides OATP1B1/1B3 was revealed [21]. As OATP1B3 is overexpressed in some malignant tumors (e.g., colon carcinomas) [22], Ncps, as inhibitors of this transporter protein, are suggested to be promising lead compounds for new drug development.

In our previous studies, *Nostoc edaphicum* CCNP1411 (Figure 1A) from the Baltic Sea was found to be a rich source of cyanopeptolins, the nonribosomal peptides with potent inhibitory activity against serine proteases [6]. In the current work, the potential of the strain to produce other bioactive metabolites was explored. The whole-genome sequence of *N*. *edaphicum* CCNP1411 has been determined, and the nostocyclopeptide biosynthetic gene cluster has been identified in the strain and characterized in silico for the first time. Furthermore, the new products of the Ncp gene cluster have been detected and their structures have been characterized by LC-MS/MS.
