Novel flax orbitide derived from genetic deletion

BACKGROUND

Flaxseed orbitides are homodetic plant cyclic peptides arising from ribosomal synthesis and post-translation modification (N to C cyclization), and lacking cysteine double bonds (Nat Prod Rep 30:108-160, 2013). Screening for orbitide composition was conducted on the flax core collection (FCC) grown at both Saskatoon, Saskatchewan and Morden, Manitoba over three growing seasons (2009-2011). Two flax (Linum usitatissimum L.) accessions 'Hollandia' (CN 98056) and 'Z 11637' (CN 98150) produce neither [1-9-NαC]-linusorb B2 (3) nor [1-9-NαC]-linusorb B3 (1). Mass spectrometry was used to identify novel compounds and elucidate their structure. NMR spectroscopy was used to corroborate structural information.

RESULTS

Experimental findings indicated that these accessions produce a novel orbitide, identified in three oxidation states having quasimolecular ion peaks at m/z 1072.6 (18), 1088.6 (19), and 1104.6 (20) [M + H]⁺ corresponding to molecular formulae C₅₇H₈₆N₉O₉S, C₅₇H₈₆N₉O₁₀S, and C₅₇H₈₆N₉O₁₁S, respectively. The structure of 19 was confirmed unequivocally as [1-9-NαC]-OLIPPFFLI. PCR amplification and sequencing of the gene coding for 18, using primers developed for 3 and 1, identified the putative linear precursor protein of 18 as being comprised of the first three amino acid residues of 3 (MLI), four conserved amino acid residues of 3 and/or 1 (PPFF), and the last two residues of 1 (LI).

CONCLUSION

Comparison of gene sequencing data revealed that a 117 base pair deletion had occurred that resulted in truncation of both 3 and 1 to produce a sequence encoding for the novel orbitide precursor of 18. This observation suggests that repeat units of flax orbitide genes are conserved and suggests a novel mechanism for evolution of orbitide gene diversity. Orbitides 19 and 20 contain MetO and MetO₂, respectively, and are not directly encoded, but are products of post-translation modification of Met present in 18 ([1-9-NαC]-MLIPPFFLI).