A novel plasmid-transcribed regulatory sRNA, QfsR, controls chromosomal polycistronic gene expression in Agrobacterium fabrum.

Plasmids are mobile DNAs that can adjust host cell functions for their own amplification and dissemination. We identified Quorum sensing flagella small RNA regulator (QfsR), a small RNA, transcribed from the virulence tumour-inducing (Ti) plasmid in the phytopathogen Agrobacterium fabrum. QfsR is widely conserved throughout RepABC plasmids carried by Rhizobiaceae. Target prediction, expression analysis and site-direct mutagenesis experiments showed that QfsR directly pairs within polycistronic mRNAs transcribed from chromosome (genes involved in flagella apparatus) and Ti plasmid (genes involved in conjugative transfer). QfsR leads to a coordinated expression of whole polycistronic mRNA molecules. Whereas a lack of QfsR represses motility, its overproduction increases the quorum sensing signal accumulation and the Ti plasmid conjugative transfer. Based on these observations, we propose QfsR as a hub connecting regulatory networks of motility and plasmid conjugative transfer. To our knowledge, QfsR is the first example of a plasmid-encoded sRNA that controls chromosomal polycistronic gene expression.