High Variability of the Electron Carrier Plastocyanin in Microalgae

Abstract

The plastocyanin content of Scenedesmus can be dramatically varied with the copper content of the culture medium. Figures as high as 7 mmol plastocyanin/mol chlorophyll are possible. Electron paramagnetic resonance (EPR) spectroscopy has been used to determine this physiological response quantitatively in intact cells having different amounts of plastocyanin. The results obtained by the EPR technique were compared with data on isolated plastocyanin determined either by spectro‐photometry or immunoelectrophoresis. Agreement was found for the amount of plastocyanin detected by the first two methods, whereas the last assay yielded data at least 25% higher on the average. Under all culture conditions a copper‐free plastocyanin precursor is present. The EPR properties of purified plastocyanin and those of cellular plastocyanin located within the thylakoids are practically identical in terms of g‐values, hyperfine splittings, signal linewidths and saturation behavior at temperatures of 12‐15 K. Our data indicate that plastocyanin is not present in a membrane‐bound form but exists as a single soluble pool. The studies have been extended to the algae Dunaliella parva and Bumilleriopsis filiformis. The first species exhibits a limited variation of plastocyanin with the copper content of the medium. Furthermore, no cytochrome c‐553 could be detected in Dunaliella even under conditions of copper deficiency. In contrast, Bumilleriopsis does not contain plastocyanin regardiess of the amount of copper offered.

Regulation by copper of the expression of plastocyanin and cytochrome c552 in Chlamydomonas reinhardi.

“…In the green alga Chlamydomonas reinhardi, the Cu2+, plastocyanin is found at a ratio of 1 per 150 chlorophyll molecules (9); whereas when Cu2+ is not available for plastocyanin synthesis S. acutus cells accumulate an immunoreactive polypeptide precursor of plastocyanin of higher apparent molecular weight (approximately 14,000) and perhaps an apoprotein (7,8). At the same time the level of soluble plastidic cytochrome C553 increases to about 1 per 100 chlorophyll molecules (9,40).…”

Biosynthesis of plastocyanin: identification of precursors

“…This is comparable to the highest plastocyanin content which can be obtained when algae are grown in the presence of 0.5-10/aM CuSO4 [6,8,9,11 ]. In copper-flee grown algae, plastocyanin is not detectable spectroscopically (detection limit: 1 plastocyanin/5000 chlorophylls).…”

“…1 B, both plastocyanin precursors can be isolated and enriched by immunoprecipitation from cell-free extracts, since they copurify with cytochrome c-553. tative immunoelectrophoresis, based on a calibration curve with pure plastocyanin [9]. Among the cytoplasmatically-synthesized chloroplast proteins so far investigated, plastocyanin differs in one important aspect: it has to cross the chloroplast envelope and the thylakoid membrane to reach its final localization.…”

“…The growth of the alga Scenedesmus acutus (strain 276-3a), the isolation of plast ocyanin, and the properties of the antiserum were as in [6,9]. Copper-deficient algae were obtained by continuous growth in a copper-free medium (<10 -~ M Cu 2+) for several months; plastocyanin was detectable neither by optical nor EPR spectroscopy.…”

“…Under copper deficiency, plastocyanin is replaced by cytochrome c-553. Plastocyanin and cytochrome c-553 form a mutually exchangeable pair functional in redox activity both in 'in vivo' and 'in vitro' assays [7,81. In an attempt to quantitatively determine this variable plastocyanin content in Scenedesmus cells, it was found that immunological methods yielded consistently higher values than spectroscopic methods (optical, EPR), which only register the holoprotein [9]. We assumed that the antiserum against plastocyanin cross-reacted with apoplastocyanin and possibly with other copper-free precursors.…”