Microcystins with Modified Adda⁵-Residues from a Heterologous Microcystin Expression System

Microcystins are hepatotoxic cyclic heptapeptides produced by some cyanobacterial species and usually contain the unusual β-amino acid 3S-amino-9S-methoxy-2S,6,8S-trimethyl-10-phenyl-4E,6E-decadienoic acid (Adda) at position-5. The full microcystin gene cluster from Microcystis aeruginosa PCC 7806 has been expressed in Escherichia coli. In an earlier study, the engineered strain was shown to produce MC-LR and [d-Asp³]MC-LR, the main microcystins reported in cultures of M. aeruginosa PCC 7806. However, analysis of the engineered strain of E. coli using semitargeted liquid chromatography with high-resolution tandem mass spectrometry (LC-HRMS/MS) and thiol derivatization revealed the presence of 15 additional microcystin analogues, including four linear peptide variants and, in total, 12 variants with modifications to the Adda moiety. Four of the Adda-variants lacked the phenyl group at the Adda-terminus, a modification that has not previously been reported in cyanobacteria. Their HRMS/MS spectra contained the product-ion from Adda at m/z 135.1168, but the commonly observed product-ion at m/z 135.0804 from Adda-containing microcystins was almost completely absent. In contrast, three of the variants were missing a methyl group between C-2 and C-8 of the Adda moiety, and their LC-HRMS/MS spectra displayed the product-ion from Adda at m/z 135.0804. However, instead of the product-ion at m/z 135.1168, these three variants gave product-ions at m/z 121.1011. These observations, together with spectra from microcystin standards using in-source fragmentation, showed that the product-ion at m/z 135.1168 found in the HRMS/MS spectra of most microcystins originated from the C-2 to C-8 region of the Adda moiety. Identification of the fragmentation pathways for the Adda side chain will facilitate the detection of microcystins containing modifications in their Adda moieties that could otherwise easily be overlooked with standard LC-MS screening methods. Microcystin variants containing Abu at position-1 were also prominent components of the microcystin profile of the engineered bacterium. Microcystin variants with Abu¹ or without the phenyl group on the Adda side chain were not detected in the original host cyanobacterium. This suggests not only that the microcystin synthase complex may be affected by substrate availability within its host organism but also that it possesses an unexpected degree of biosynthetic flexibility.