Evolution of Anabaenopeptin Peptide Structural Variability in the Cyanobacterium Planktothrix

Area of Science:

• Microbiology
• Environmental Science
• Biochemistry

Background:

• Cyanobacteria, particularly Planktothrix species, are key players in harmful algal blooms.
• These blooms are associated with toxic microcystins and other bioactive peptides like anabaenopeptins (APs).
• Anabaenopeptins are nonribosomally synthesized cyclic hexapeptides with variable exocyclic amino acids.

Purpose of the Study:

• To investigate the distribution and recombination of the anabaenopeptin synthesis gene cluster (apnA-E) in Planktothrix spp.
• To compare the apnA-E gene cluster across ecologically diverse Planktothrix strains from various habitats.
• To understand the evolutionary dynamics and horizontal gene transfer of AP biosynthesis genes.

Main Methods:

• Comparative genomics of 125 Planktothrix strains, including whole-genome sequencing of ten ecologically divergent strains.
• PCR amplification of the apn gene cluster to quantify gene distribution.
• Phylogenetic analysis and genetic analysis of the apnA gene's A1 domain.

Main Results:

• The apnA-E genes were absent in P. pseudagardhii, P. mougeotii, and P. tepida strains (Lineage 3).
• Shallow water P. agardhii/P. rubescens strains (Lineage 1) showed variable presence of the apn genes, with evidence of gene cluster deletion in some.
• Deep water ecotype strains (Lineage 2) consistently possessed the apnA-E genes, indicating their stable presence in this lineage.
• High similarity of apn genes between Planktothrix and Microcystis suggests horizontal gene transfer.
• Analysis of the apnA A1 domain revealed four genotype groups linked to specific amino acid production (Arginine, Tyrosine, Lysine), driven by recurrent point mutations.

Conclusions:

• The presence and distribution of the anabaenopeptin gene cluster vary significantly within Planktothrix species, correlating with ecotype and habitat.
• Horizontal gene transfer likely played a role in the evolution of anabaenopeptin biosynthesis in Planktothrix.
• The diversity in AP structure is driven by adaptive evolution of the apnA gene, allowing for varied exocyclic amino acid incorporation.