The genome of Xanthomonas campestris pv. vesicatoria encodes a constitutively expressed small RNA, which we designate PtaRNA1, "Plasmid transferred anti-sense RNA". It exhibits all hallmarks of a novel RNA antitoxin that proliferates by frequent horizontal transfer. It shows an erratic phylogenetic distribution with occurrences on chromosomes in a few individual strains distributed across both beta- and gamma-proteobacteria. Moreover, a homologous gene located on plasmid pMATVIM-7 of Pseudomonas aeruginosa is found. All ptaRNA1 homologs are located anti-sense to a putative toxin, which in turn is never encountered without the small RNA. The secondary structure of PtaRNA1, furthermore, is very similar to that of the FinP anti-sense RNA found on F-like plasmids in Escherichia coli.