In some cyanobacteria and eukaryotic algae, cytochrome c-553 (c-552) and plastocyanin function as alternative electron carriers between the cytochrome b-f complex and Photosystem I. In these organisms plastocyanin is the electron carrier under copper-replete conditions, and cytochrome c-553 is the electron carrier during copper deprivation. In this paper we report the cloning, sequencing and transcriptional analysis of the genes for cytochrome c-553 and plastocyanin from sp. PCC 7120. The gene for cytochrome c-553 encodes a preprotein containing 111 amino acids with a predicted -terminal transit peptide sequence of 25 amino acids. The gene for plastocyanin encodes a preprotein containing 139 amino acids with a -terminal transit peptide sequence of 34 amino acids. RNA transcript analyses indicate that the expression of the genes for cytochrome c-553 (petl) and plastocyanin (petE) are regulated in reciprocal ways in response to copper concentration. In copper-replete conditions, petJ is expressed at very low levels, but is transcribed at high levels under copper deprivation; petE is down-regulated in the absence of copper, but is rapidly up-regulated when copper is added back to the medium.


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