Fig. 1.

Distribution of plasmid lengths among mobility classes of plasmids. Plasmid length data are taken from PLSDB (retrieved 1 March 2023 [237]) and the distribution is derived by kernel density estimation in R [238] with the ggplot2 library [239]. The top panel shows all plasmids in the database; the bottom four panels show plasmids from the four bacterial families with the most sequences in the database. The graph was cut off at 300 000 bp for reasons of scale: 1844 plasmids in the database (5.34% of the total) are longer than the limit; the longest plasmid in the database is 4 605 385 bp. The plasmid sequences in PLSDB have been annotated with MOB-typer [240] to identify putative relaxases* or conjugative genes (see the explanation of these terms in the text). Those plasmids with neither are nontransmissible, those with both are conjugative, and those with only the relaxase are mobilizable; note that this means that those mobilizable plasmids with an oriT but no relaxase are not recognized as mobilizable. The 'weird’ class includes those plasmids that had conjugative genes but no relaxase: these would be nontransmissible, but nonetheless have all the genes for a secretion system and mating pair formation. It is possible that these are misidentified conjugative plasmids, which have an unknown relaxase, or misidentified mobilizable or nontransmissible plasmids, which do not actually have conjugative genes.