Translation of merD in Tn21

All four sequenced examples of the mercury resistance (mer) operon of gram-negative bacteria have a promoter-distal reading frame, merD, whose removal has little effect on the resistance phenotype and whose translation has not previously been observed. Using merD-lacZ protein fusions, we show that merD is translated. However, Hg(II)-induced merD expression, as measured by beta-galactosidase activity and immunoblotting, is 10- to 15-fold lower than that of fusions to the gene immediately preceding it, merA.