Genetic variation of adenylation domains of the anabaenopeptin synthesis operon and evolution of substrate promiscuity.

Anabaenopeptins (AP) are bioactive cyclic hexapeptides synthesized nonribosomally in cyanobacteria. APs are characterized by several conserved motifs, including the ureido bond, N-methylation in position 5, and d-Lys in position 2. All other positions of the AP molecule are variable, resulting in numerous structural variants. We have identified a nonribosomal peptide synthetase (NRPS) operon from Planktothrix agardhii strain CYA126/8 consisting of five genes (apnA to apnE) encoding six NRPS modules and have confirmed its role in AP synthesis by the generation of a mutant via insertional inactivation of apnC. In order to correlate the genetic diversity among adenylation domains (A domains) with AP structure variation, we sequenced the A domains of all six NRPS modules from seven Planktothrix strains differing in the production of AP congeners. It is remarkable that single strains coproduce APs bearing either of the chemically divergent amino acids Arg and Tyr in exocyclic position 1. Since the A domain of the initiation module (the ApnA A₁ domain) has been proposed to activate the amino acid incorporated into exocyclic position 1, we decided to analyze this domain both biochemically and phylogenetically. Only ApnA A₁ enzymes from strains producing AP molecules containing Arg or Tyr in position 1 were found to activate these two chemically divergent amino acids in vitro. Phylogenetic analysis of apn A domain sequences revealed that strains with a promiscuous ApnA A₁ domain are derived from an ancestor that activates only Arg. Surprisingly, positive selection appears to affect only three codons within the apnA A₁ gene, suggesting that this remarkable promiscuity has evolved from point mutations only.