EXPRESSION OF THE ESCHERICHIA-COLI CHROMOSOMAL ARS OPERON

A chromosomally located operon (ars) of Escherichia coil has been prev iously shown to be functional in arsenic detoxification. DNA sequencin g revealed three open reading frames homologous to the arsR, arsB, and arsC open reading frames of plasmid-based arsenic resistance operons isolated from both E. coli and staphylococcal species. To examine the outline of transcriptional regulation of the chromosomal ars operon, s everal transcriptional fusions, using the luciferase-encoding luxAB ge nes of Vibrio harveyi, were constructed. Measurement of the expression of these gene fusions demonstrated that the operon was rapidly induce d by sodium arsenite and negatively regulated by the trans-acting arsR gene product. Northern blotting and primer extension analyses reveale d that the chromosomal ars operon is most likely transcribed as a sing le mRNA of approximately 2100 nucleotides in length arid processed int o two smaller mRNA products in a manner similar to that found in the E . coli R773 plasmid-home ars operon. However, transcription was found to initiate at a position that is relatively further upstream of the i nitiation codon of the arsR coding sequence than that determined for t he E. coli R773 plasmid's ans operon.