Expression of merA, trxA, amoA, and hao in continuously cultured Nitrosomonas europaea cells exposed to cadmium sulfate additions

Abstract

The effects of CdSO(4) additions on the gene expressions of a mercury reductase, merA, an oxidative stress protein, trxA, the ammonia-monooxygenase enzyme (AMO), amoA, and the hydroxylamine oxidoreductase enzyme (HAO), hao, were examined in continuously cultured N. europaea cells. The reactor was fed 50 mM NH(4)+ and was operated for 78 days with a 6.9 days hydraulic retention time. Over this period, six successive batch additions of CdSO(4) were made with increasing maximum concentrations ranging from 1 to 60 microM Cd(2+). The expression of merA was highly correlated with the level of Cd(2+) within the reactor (Rs = 0.90) with significant up-regulation measured at non-inhibitory Cd(2+) concentrations. Cd(2+) appears to target AMO specifically at lower concentrations and caused oxidative stress at higher concentrations, as indicated by the SOURs (specific oxygen uptake rates) and the up-regulation of trxA. Since Cd(2+) inhibition is irreversible and amoA was up-regulated in response to Cd(2+) inhibition, it is hypothesized that de novo synthesis of the AMO enzyme occurred and was responsible for the observed recovery in activity. Continuously cultured N. europaea cells were more resistant to Cd(2+) inhibition than previously examined batch cultured cells due to the presence of Mg(2+) and Ca(2+) in the growth media, suggesting that Cd(2+) enters the cell through Mg(2+) and Ca(2+) import channels. The up-regulation of merA during exposure to non-inhibitory Cd(2+) levels indicates that merA is an excellent early warning signal for Cd(2+) inhibition.