Two 'calcium-irreparable' acid-sensitive mutants were identified after mutagenizing Rhizobium leguminosarum bv. viciae and Sinorhizobium meliloti with Tn5. Each mutant contains a single copy of the transposon which, inserted within the actP gene, prevents expression of a P-type ATPase that belongs to the CPx heavy metal-transporting subfamily. Here, we show that both actP-knockout mutants show sensitivity to copper; omission of this heavy metal from low pH-buffered media restores acid tolerance to these strains. Furthermore, complementation of the mutant phenotype requires only the actPgene. An actP-gusA fusion in R. leguminosarum was transcriptionally regulated by copper in a pH-dependent manner.Downstream to actP in both organisms is the hmrR gene that encodes a heavy metal-responsive regulator (HmrR) that belongs to the merR class of regulatory genes. Insertional Inactivation of hmrR abolished transcriptional activation of actP by copper ions and increased the basal level of its expression in their absence. These observations suggest that HmrR can regulate actP transcription positively and negatively. We show that copper homeostasis is an essential mechanism for the acid tolerance of these root nodule bacteria since it prevents this heavy metal from becoming overtly toxic in acidic conditions.