The effect of copper on the levels of plastocyanin (PC) and cytochrome c553 (cyt-c)-specific transcripts from Anabaena sp. PCC 7937 was investigated. The addition of copper resulted in a marked increase in PC mRNA levels, and a decrease in cyt c mRNA levels. Thus the functional exchange between PC and cyt c seems to be regulated at the mRNA level. The copper-dependent increase in PC and decrease in cyt c mRNA levels was abolished when chloramphenicol was added to the cells. This suggests that de novo synthesis of at least one trans-acting element is required to regulate PC and cyt c mRNA levels. Both PC and cyt c mRNA stability was found to be unaltered under varying Cu2+ regimes. This leads to the conclusion that expression of both genes is regulated at the level of initiation of transcription.