Abstract
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.
Highlights
The arsenical resistance operon of the conjugative plasmid R773 encodes an ATP-driven anion extrusion pump, conferring bacterial resistance to arsenicals
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
From the genetic evidence [7, 22] and from the nucleotide sequence [6], the oxyanion pump was predicted to be composed of a complex of the 63-kilodalton ArsA and the 45.5-kilodalton ArsB proteins
Summary
The arsenical resistance (ars) operon of the conjugative plasmid R773 encodes an ATP-driven anion extrusion pump, conferring bacterial resistance to arsenicals. 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. The arsenical resistance (ars) operon of resistance plasmid R773 confers resistance to arsenite, arsenate, and antimonite on Escherichia coli cells by the synthesis of an anion pump [23] This unique oxyanion-translocating ATPase, induced by the presence of its substrates, mediates their active extrusion from cells with energy derived from ATP [18, 22, 23, 28]. Quence of the arsR gene has been determined, and its product, the ArsR protein, has been identified
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