Analysis of defence systems and a conjugative IncP‐1 plasmid in the marine polyaromatic hydrocarbons‐degrading bacterium <i>Cycloclasticus</i> sp. 78‐ME

<jats:title>Summary</jats:title><jats:p>Marine prokaryotes have evolved a broad repertoire of defence systems to protect their genomes from lateral gene transfer including innate or acquired immune systems and infection‐induced programmed cell suicide and dormancy. Here we report on the analysis of multiple defence systems present in the genome of the strain <jats:italic>Cycloclasticus</jats:italic> sp. 78‐ME isolated from petroleum deposits of the tanker <jats:italic>‘Amoco Milford Haven’</jats:italic>. <jats:italic>Cycloclasticus</jats:italic> are ubiquitous bacteria globally important in polyaromatic hydrocarbons degradation in marine environments. Two ‘defence islands’ were identified in 78‐ME genome: the first harbouring CRISPR‐Cas with toxin‐antitoxin system, while the second was composed by an array of genes for toxin‐antitoxin and restriction‐modification proteins. Among all identified spacers of CRISPR‐Cas system only seven spacers match sequences of phages and plasmids. Furthermore, a conjugative plasmid p7ME01, which belongs to a new IncP‐1θ ancestral archetype without any accessory mobile elements was found in 78‐ME. Our results provide the context to the co‐occurrence of diverse defence mechanisms in the genome of <jats:italic>Cycloclasticus</jats:italic> sp. 78‐ME, which protect the genome of this highly specialized PAH‐degrader. This study contributes to the further understanding of complex networks established in petroleum‐based microbial communities.</jats:p>