The reasons for the most recent outbreaks remain unclear, and Y. In recent years, the number of outbreaks in the farmed Atlantic salmon population has substantially increased. ruckeri among Atlantic salmon was described in Norway in 1985, and this was successfully treated with antibiotics (Sparboe et al., 1986). The first report of a disease outbreak caused by Y. For a long time, ERM has played a minor role in Norway, with only a few outbreaks per year (Hjeltnes et al., 2017). ruckeri serotype 1 belonging to biotype 1, characterized as motile with phospholipase activity (Romalde and Toranzo, 1993). The majority of the ERM outbreaks are caused by the highly pathogenic Y. Despite the availability of vaccines, yersiniosis outbreaks still occur in fish farms (Ormsby et al., 2016). ruckeri is mostly transmitted through contact with carrier fish (Busch, 1978 Stevenson and Airdrie, 1984). This bacterium contributes to enormous economic losses in aquaculture throughout the world. ruckeri is a fish pathogen causing enteric redmouth disease (ERM), mainly in salmonids (Bullock et al., 1978 Busch, 1978). pseudotuberculosis are responsible for a broad range of diseases ranging from mild gastroenteritis to life-threatening septicemia (Bottone, 1997). pseudotuberculosis are transmitted via ingestion of contaminated food or water (Bottone, 1997 Perry and Fetherston, 1997 Jalava et al., 2006). pestis spreads through fleabites or aerosols, whereas Y. pseudotuberculosis, well-known human enteropathogens. pestis, the causative agent of bubonic and pneumonic plague, is one of the most virulent organisms known (Chauhan et al., 2016). Although the human pathogens within the genus are closely related to each other, they cause diverse diseases. The genus Yersinia consists of 17 different species (Reuter et al., 2014 Savin et al., 2014). As the tra operons of pYR3, although misplaced on the chromosome during the genome assembly process, were demonstrated to have an effect on virulence, and type IV pili are virulence factors in many bacteria, we suggest that pYR4 directly contributes to Y. Thus, PacBio sequencing or similar long-read methods should always be preferred for de novo genome sequencing. Placing plasmid sequences on the chromosome can result in erroneous biological conclusions. ruckeri ATCC29473 genome assemblies, we mapped the entire pYR3 plasmid sequence. ruckeri plasmids and genomes, which were sequenced by second and the third generation sequencing technologies, showed errors in second generation sequencing assemblies. Comparative analysis of pYR4 and other Y. Our study highlights the power of PacBio sequencing technology for identifying mis-assembled segments of genomic sequences. In contrast to other Yersinia plasmids, pYR4 cannot be cured at elevated temperatures. pYR4, as well as the related plasmid pYR3, comprises operons that encode for type IV pili and for a conjugation system ( tra). ruckeri NVH_3758 genome, indicating its relatively recent horizontal acquisition. The G+C content of pYR4 is higher than that of the Y. The unique regions contain, among others genes, a large number of mobile genetic elements and two partitioning systems. Thirty-one percent of the pYR4 sequence is unique compared to other Y. Like the well-known pYV plasmid of human pathogenic Yersiniae, pYR4 is a member of the IncFII family. ruckeri isolate, sequenced using PacBio SMRT technology. Here, we report and analyze the complete DNA sequence of pYR4, a plasmid from a highly pathogenic Norwegian Y. ruckeri have been identified and studied in detail. Despite its importance, only a few virulence factors of Y. Enteric redmouth disease caused by the pathogen Yersinia ruckeri is a significant problem for fish farming around the world.