Dynamic interplay between two copper-titrating components in the transcriptional regulation of cyt c6.

Abstract

The algal plastidic cytochrome c (cyt c6) is a biochemical equivalent of the copper‐containing protein plastocyanin in photosynthetic electron transfer. But generally, cyt c6 accumulates and functions only under conditions (e.g. Cu‐deficiency) where holoplastocyanin cannot be synthesized. In studying the regulation of Chlamydomonas reinhardtii cyt c6 expression by Cu we have determined that repression of cyt c6 accumulation occurs at the transcriptional level, and specifically in response to Cu as the metal ion regulator. Complete and sustained repression of cyt c6 transcription requires approximately 9 ×10(6) Cu ions in the medium/cell. Based on the estimated plastocyanin content of algal cells (8 ×10(6) molecules/cell) and the observation that lower ratios of Cu per cell result in only transient repression of cyt c6 transcription, we propose that Cu‐dependent transcriptional repression of the gene encoding cyt c6 requires a Cu‐binding factor which is titrated by Cu only after the alternate electron transfer catalyst, plastocyanin, has accumulated to the stoichiometry required for photosynthesis. The precise and highly metal‐specific, autoregulatory control of cyt c6 levels‐‐directly by Cu, and indirectly by holoplastocyanin‐‐is in keeping with the functional role of cyt c6 as an alternate, although perhaps less preferred, electron transfer catalyst.

In Vivo Competition between Plastocyanin and a Copper-Dependent Regulator of the Chlamydomonas reinhardtii Cytochrome c6 Gene

“…This level of plastocyanin is presumably optimal for photosynthesis and is likely determined by intemal factors that govern the stoichiometry of the various thylakoid membrane components. Thus, further increases in medium copper (>8 x 106_9 x 106 cupric ions/cell) do not result in additional plastocyanin accumulation (21).…”

“…The level of plastocyanin in C reinhardtii is proportional to the amount of copper available until the plastocyanin content reaches approximately 8 x 106 molecules/cell (21). This level of plastocyanin is presumably optimal for photosynthesis and is likely determined by intemal factors that govern the stoichiometry of the various thylakoid membrane components.…”

“…In the case of Cyt c6, copper-dependent repression of its accumulation in C reinhardtii is achieved primarily via transcriptional control (21), with little or no contribution from posttranscriptional processes (such as differential mRNA stability) (10). The transcriptional response of the Cyt c6 gene occurs independently of the photosynthetic process (19).…”

How do plants respond to copper deficiency?

“…10,11 In addition plants respond to copper deficiency by expressing the alternative iron proteins which complement the function of copper proteins. 12 Among photosynthetic eukaryotes, the unicellular green alga, Chlamydomonas reinhardtii, is best studied with respect to the molecular mechanism of copper homeostasis. 13 Under low copper conditions, Chlamydomonas degrades plastocyanin and its function is replaced by cytchrome c 6 containing iron.…”

“…13 Under low copper conditions, Chlamydomonas degrades plastocyanin and its function is replaced by cytchrome c 6 containing iron. 12 A transcriptional factor, Crr1, plays a pivotal role in switching the photosynthetic machinery and probably also in copper sensing. 14 However, higher plants are unlikely to encode the functional homolog of the algal cytochrome c 6 .…”