The expression of the petE gene encoding plastocyanin (PC) in the prokaryote Prochlorothrix hollandica is dependent on the presence of copper in the medium. PC protein and petE mRNA are detectable only under copper (Cu) replete conditions, suggesting that control of PC accumulation can occur at the level of transcription or transcript stability. Addition of Cu (0.3 microM) to log-phase Cu-deficient cultures yields accumulation of PC to detectable levels within 12 h; transfer of Cu-replete cells to Cu-deficient medium results in a slow decrease in the level of protein likely due to dilution by cell growth. By contrast, addition of high-affinity Cu-specific chelators to rapidly deplete Cu-grown cells of copper yields a rapid loss of PC with 2 h. These data suggest that Cu-free apoPC is turned over rapidly by proteolysis. Overall, these data demonstrate that regulation of PC levels as Cu levels change involve events occurring at the level of both RNA and protein turnover.