The DNA sequence has been determined for a 3.8-kilobase region which encodes the mercury resistance (mer) operon of the IncFII plasmid NR1. The sequence reveals four open reading frames which could encode proteins of 12,391, 9,429, 14,965, and 58,781 daltons. On the basis of their sizes, amino acid compositions, hydropathicities, and estimated isoelectric points, the peptides encoded by these open reading frames correspond to the four previously described Hg-inducible proteins detected in minicells carrying mer+ plasmids. The NR1 mer locus is 63.4% GC overall, and the Hg(II) reductase protein sequence is 90% homologous to that of Tn501. The region encoding the merR (positive regulatory) function has three open reading frames. The smallest of these possible merR peptides (6,457 daltons) begins approximately 280 bp to the right of the adjacent IS1b and reads towards the structural genes of the mer operon. The next largest reading frame (13,139 daltons) in the merR region begins 37 bp to the left of the beginning of the smallest peptide and also reads towards the structural genes. The largest reading frame (15,907 daltons) in the merR region lies on the complementary strand and reads away from the structural genes towards IS1b. Although attempts to visualize the merR gene product were not successful, in vitro mutagenesis allows us to eliminate the largest reading frame as a merR candidate. We were also able to show that approximately 50% of the smallest detectable mer peptide (9,429 daltons) is located in the periplasm.