**Marta Cegłowska 1, Karolia Szubert 2, Ewa Wieczerzak 3, Alicja Kosakowska <sup>1</sup> and Hanna Mazur-Marzec 2,\***


Received: 31 July 2020; Accepted: 25 August 2020; Published: 27 August 2020

**Abstract:** Cyanobactins are a large family of ribosomally synthesized and post-translationally modified cyanopeptides (RiPPs). Thus far, over a hundred cyanobactins have been detected in different free-living and symbiotic cyanobacteria. The majority of these peptides have a cyclic structure. The occurrence of linear cyanobactins, aeruginosamides and virenamide, has been reported sporadically and in few cyanobacterial taxa. In the current work, the production of cyanobactins by *Limnoraphis* sp. CCNP1324, isolated from the brackish water Baltic Sea, has been studied for the first time. In the strain, eighteen new aeruginosamide (AEG) variants have been detected. These compounds are characterized by the presence of prenyl and thiazole groups. A common element of AEGs produced by *Limnoraphis* sp. CCNP1324 is the sequence of the three C-terminal residues containing proline, pyrrolidine and methyl ester of thiazolidyne-4-carboxylic acid (Pro-Pyr-TzlCOOMe) or thiazolidyne-4-carboxylic acid (Pro-Pyr-TzlCOOH). The aeruginosamides with methylhomotyrosine (MeHTyr1) and with the unidentified *N*-terminal amino acids showed strong cytotoxic activity against human breast cancer cells (T47D).

**Keywords:** cyanobacteria; aeruginosamides; *Limnoraphis*; cytotoxicity
