To settle or to move? The interplay between two classes of cyclic lipopeptides in the biocontrol strain Pseudomonas CMR12a

Pseudomonas CMR12a is a biocontrol strain that produces phenazine antibiotics and as yet uncharacterized cyclic lipopeptides (CLPs). The CLPs of CMR12a were studied by chemical structure analysis and in silico analysis of the gene clusters encoding the nonribosomal peptide synthetases responsible for CLP biosynthesis. CMR12a produces two different classes of CLPs: orfamides B, D and E, whereby the latter two represent new derivatives of the orfamide family, and sessilins A-C. The orfamides are made up of a 10 amino acid peptide coupled to a β-hydroxydodecanoyl or β-hydroxytetradecanoyl fatty acid moiety, and are related to orfamides produced by biocontrol strain P. protegens Pf-5. The sessilins consist of an 18 amino acid peptide linked to a β-hydroxyoctanoyl fatty acid, and differ in one amino acid from tolaasins, toxins produced by the mushroom pathogen P. tolaasii. CLP biosynthesis mutants were constructed and tested for biofilm formation and swarming motility. Orfamides appeared indispensable for swarming, while sessilin mutants showed reduced biofilm formation, but enhanced swarming motility. The interplay between the two classes of CLPs fine-tunes these processes. The presence of sessilins in wild type CMR12a interferes with swarming by hampering the release of orfamides and by co-precipitating orfamides to form a white-line-in agar.