Evolution of a Plasmid Regulatory Circuit Ameliorates Plasmid Fitness Cost

ABSTRACTPlasmids play a major role in rapid adaptation of bacteria by facilitating horizontal transfer of diverse genes, most notably those conferring antibiotic resistance. While most plasmids that replicate in a broad range of bacteria also persist well in diverse hosts, there are exceptions that are poorly understood. We investigated why a broad-host range plasmid, pBP136, originally found in clinicalBordetella pertussisisolates, quickly became extinct in laboratoryEscherichia colipopulations. Through experimental evolution we found that inactivation of a previously uncharacterized plasmid gene,upf31, drastically improved plasmid maintenance inE. coli. This gene inactivation resulted in decreased transcription of the global plasmid regulators (korA,korB,andkorC)and numerous genes in their regulons. It also caused transcriptional changes in many chromosomal genes primarily related to metabolism.In silicoanalyses suggested that the change in plasmid transcriptome may be initiated by Upf31 interacting with the plasmid regulator KorB. Expression ofupf31 in transnegatively affected persistence of pBP136Δupf31as well as the closely related archetypal IncP-1β plasmid R751, which is stable inE. coliand natively encodes a truncatedupf31allele. Our results demonstrate that while theupf31allele in pBP136 might advantageously modulate gene expression in its original host,B. pertussis, it has harmful effects inE. coli. Thus, evolution of a single plasmid gene can change the range of hosts in which that plasmid persists, due to effects on the regulation of plasmid gene transcription.