Conjugation and transposon mutagenesis of HGB2511, the bacterial symbiont of the nematode (India).

Symbiosis, the beneficial interactions between two organisms, is a ubiquitous feature of all life on Earth, including associations between animals and bacteria. However, the specific molecular and cellular mechanisms which underlie the diverse partnerships formed between animals and bacteria are still being explored. Entomopathogenic nematodes transport bacteria between insect hosts, together they kill the insect, and the bacteria consume the insect and serve as food source for the nematodes. These nematodes, including those in the genus, are effective laboratory models for studying the molecular mechanisms of symbiosis because of the natural partnership they form with bacteria and their straightforward husbandry. nematodes and their symbiotic bacteria are being developed as a genetic model pair for studying symbiosis. Our goal in this project was to begin to identify bacterial genes that may be important for symbiotic interactions with the nematode host. Towards this end, we adapted and optimized a protocol for delivery and insertion of a promoter-probe transposon for use in the symbiont, HGB2511 (Cao et al., 2022). We assessed the frequencies at which we obtained exconjugants, metabolic auxotrophic mutants, and active promoter- fusions. Our data indicate that the Tn transposon inserted relatively randomly based on the finding that 4.7% of the mutants exhibited an auxotrophic phenotype. Promoter-fusions with the transposon-encoded , which resulted in expression of β-galactosidase activity, occurred in 47% of the strains. To our knowledge, this is the first mutagenesis protocol generated for this bacterial species, and will facilitate the implementation of large scale screens for symbiosis and other phenotypes of interest in .