Evolution of a Plasmid Regulatory Circuit Ameliorates Plasmid Fitness Cost.

Plasmids 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 clinical isolates, quickly became extinct in laboratory populations. Through experimental evolution we found that inactivation of a previously uncharacterized plasmid gene, , drastically improved plasmid maintenance in . This gene inactivation resulted in decreased transcription of the global plasmid regulators (, and and numerous genes in their regulons. It also caused transcriptional changes in many chromosomal genes primarily related to metabolism. analyses suggested that the change in plasmid transcriptome may be initiated by Upf31 interacting with the plasmid regulator KorB. Expression of negatively affected persistence of pBP136Δ as well as the closely related archetypal IncP-1β plasmid R751, which is stable in and natively encodes a truncated allele. Our results demonstrate that while the allele in pBP136 might advantageously modulate gene expression in its original host, , it has harmful effects in . 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.