Résumé
Background: DNA methylation is an epigenetic mechanism involved in the pathogenicity of severalmajor bacterial pathogens. It can decrease the affinity of some transcriptional regulators to theirbinding site, leading to sub‐populations expressing or not various genes, depending on the DNAmethylation state. Dam DNA methyltransferase is widespread in Gammaproteobacteria andmethylates the adenine of GATC sites.Objectives: The role of Dam was investigated in Photorhabdus luminescens during its symbiosis witha soil nematode and during its pathogenic stage in insects.Methods: SMRT sequencing (PacBio), which allows identification of the DNA methylation of thewhole genome (methylome), RNAseq and phenotypic analysis were performed in a P. luminescensstrain overexpressing Dam.Results: Dam overexpression caused a decrease in motility whereas it increased biofilm formation.While symbiosis ability of the Dam overexpressing strain was not significantly different from that of acontrol strain, the nemato‐bacterial complex displayed an impaired pathogenicity in insect, as alsoobserved after direct insect injection of the bacteria alone. Transcriptomic analysis revealed that theobserved phenotypes were related to differences at the transcriptional level. More than 99% of theGATC sites of the genome were found methylated and DNA methylation levels did not change overgrowth kinetics. However, the Dam‐overexpressing strain displayed more methylated GATC sitesthan the control and most of these sites were located in promoter regions. These sites may beinvolved in the observed differences in phenotypes and gene expression and provide