Pseudovibriamides from <i>Pseudovibrio</i> marine sponge bacteria promote flagellar motility via transcriptional modulation

<i>Pseudovibrio</i> α-Proteobacteria have been repeatedly isolated from marine sponges and proposed to be beneficial to the host. Bacterial motility is known to contribute to host colonization. We have previously identified pseudovibriamides A and B, produced in culture by <i>Pseudovibrio brasiliensis</i> Ab134, and shown that pseudovibriamide A promotes flagellar motility. Pseudovibriamides are encoded in a hybrid nonribosomal peptide synthetase-polyketide synthase gene cluster that also includes several accessory genes. Pseudovibriamide A is a linear heptapeptide and pseudovibriamide B is a nonadepsipeptide derived from pseudovibriamide A. Here, we define the borders of the pseudovibriamides gene cluster, assign function to biosynthetic genes using reverse genetics, and test the hypothesis that pseudovibriamides impact motility by modulating gene transcription. RNA-sequencing transcriptomic analyses of strains having different compositions of pseudovibriamides suggested that both pseudovibriamides A and B affect genes potentially involved in motility, and that a compensatory mechanism is at play in mutants that produce only pseudovibriamide A, resulting in comparable flagellar motility as the wild type. The data gathered suggest that pseudovibriamides A and B have opposite roles in modulating a subset of genes, with pseudovibriamide B having a primary effect in gene activation, and pseudovibriamide A on inhibition. Finally, we observed many differentially expressed genes (up to 29% of the total gene number) indicating that pseudovibriamides have a global effect on transcription that goes beyond motility.IMPORTANCEMarine sponges are found throughout the oceans from tropical coral reefs to polar sea floors, playing crucial roles in marine ecosystems. <i>Pseudovibrio</i> bacteria have been proposed to contribute to sponge health. We have previously shown that pseudovibriamides produced by <i>Pseudovibrio brasiliensis</i> promote bacterial motility, a behavior that is beneficial to bacterial survival and host colonization. The gene cluster that encodes pseudovibriamide biosynthesis is found in two-thirds of <i>Pseudovibrio</i> genomes. This gene cluster is also present in <i>Pseudomonas</i> bacteria that interact with terrestrial plants and animals. Here, we first assign functions to pseudovibriamide biosynthetic genes using reverse genetics. We then show that pseudovibriamides play a major role in transcriptional regulation, affecting up to 29% of <i>P. brasiliensis</i> genes, including motility genes. Thus, this work gives insights into pseudovibriamide biosynthesis and provides evidence that they are signaling molecules relevant to bacterial motility and to other yet-to-be-identified phenotypes.