A novel plasmid-transcribed regulatory sRNA, QfsR, controls chromosomal polycistronic mRNAs in Agrobacterium tumefaciens

Plasmids are mobile DNAs that adjust host cell functions for their own amplification and dissemination. We identified QfsR, a small RNA, transcribed from the 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 show that QfsR directly pairs within polycistronic mRNAs transcribed from chromosomes (involved in flagella apparatus and succinoglycan biosynthesis) and Ti plasmid (involved in conjugative transfer). QfsR leads to a coordinated expression of whole polycistronic mRNA molecules. Whereas a lack of QfsR induces motility and reduces succinoglycan production, 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, succinoglycan biosynthesis and plasmid conjugative transfer. To our knowledge, QfsR is the first example of a plasmid-encoded sRNA that controls chromosomal polycistronic mRNAs.