Chloroplasts contain homologues to the proteolytic and regulatory subunits of bacterial ATP-dependent Clp protease. We tested the effects of light and temperature on the expression of ClpC, the chloroplastic homologue of the regulatory subunit. ClpC mRNA was present in all tissues of pea seedlings, most abundantly in leaves. Higher levels of the message were found in green leaves than in etiolated ones. Exposure of etiolated seedlings to light resulted in further accumulation of the transcript. Similarly, ClpC protein level was lower in etiolated leaves, and increased upon exposure to light. Transferring seedlings from 25 degrees C to either 17 or 37 degrees C resulted in a decrease in both ClpC mRNA and protein, with the lower temperature being the most effective.