Differential mRNA stability controls relative gene expression within the plasmid-encoded arsenical resistance operon

The arsenical resistance (ars) operon of the conjugative plasmid R773 encodes an ATP-driven anion extrusion pump, conferring bacterial resistance to arsenicals. The operon contains a regulatory gene, arsR, and three structural genes, arsA, arsB, and arsC. The hydrophilic ArsA and ArsC proteins are produced in large amounts, but the hydrophobic ArsB protein, an integral membrane polypeptide, is synthesized in limited quantities. Northern (RNA-DNA) hybridizations provide evidence that the inducible operon is regulated at the level of transcription. The genes were transcribed in the presence of an inducer (arsenite) as a single polycistronic mRNA with an approximate size of 4.4 kilobases (kb). This transcript was processed to generate relatively stable mRNA species: one of 2.7 kb, encoding the ArsR and ArsA proteins, and a second of 0.5 kb, encoding the ArsC protein. Segmental differences in stability within the polycistronic transcript are proposed to account for the differential expression of the ars genes. In addition, analysis of the mRNA structure at the 5' end of arsB suggests a potential translational block to the synthesis of this membrane protein.