The biosynthesis of Caryophyllaceae-like cyclic peptides in Saponaria vaccaria L. from DNA-encoded precursors.

Cyclic peptides (CPs) are produced in a very wide range of taxa. Their biosynthesis generally involves either non-ribosomal peptide synthases or ribosome-dependent production of precursor peptides. Plants within the Caryophyllaceae and certain other families produce CPs which generally consist of 5-9 proteinogenic amino acids. The biological roles for these CPs in the plant are not very clear, but many of them have activity in mammalian systems. There is currently very little known about the biosynthesis of CPs in the Caryophyllaceae. A collection of expressed sequence tags from developing seeds of Saponaria vaccaria was investigated for information about CP biosynthesis. This revealed genes that appeared to encode CP precursors which are subsequently cyclized to mature CPs. This was tested and confirmed by the expression of a cDNA encoding a putative precursor of the CP segetalin A in transformed S. vaccaria roots. Similarly, extracts of developing S. vaccaria seeds were shown to catalyze the production of segetalin A from the same putative (synthetic) precursor. Moreover, the presence in S. vaccaria seeds of two segetalins, J [cyclo(FGTHGLPAP)] and K [cyclo(GRVKA)], which was predicted by sequence analysis, was confirmed by liquid chromatography/mass spectrometry. Sequence analysis also predicts the presence of similar CP precursor genes in Dianthus caryophyllus and Citrus spp. The data support the ribosome-dependent biosynthesis of Caryophyllaceae-like CPs in the Caryophyllaceae and Rutaceae.