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Studies of Sulfate Utilization of Algae

15. Enzymes of Assimilatory Sulfate Reduction in Euglena and Their Cellular Localization ¹

^a Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02154

Crude extracts of wild-type Euglena grown in the light (WTL) or in the dark (WTD) and a mutant lacking detectable plastid DNA (W₃BUL) contain adenosine 5'-phosphosulfate (APS) sulfotransferase. Isotope dilution experiments indicate that adenosine 3'-phosphate 5'-phosphosulfate (PAPS) sulfotransferase is absent.

Thiosulfonate reductase, requiring addition of NADH or NADPH but not ferredoxin, and O-acetyl-L-serine sulfhydrylase, the two other enzymes of the bound intermediate pathway of assimilatory sulfate reduction, are also present. Increasing levels of all three enzymes were found in WTL, WTD, and W₃BUL during logarithmic growth but the various activities were similar at comparable stages of growth in all three types of cell.

These results show that the three enzymes are not coded in the chloroplast DNA and are not restricted to Euglena cells having fully developed chloroplasts. Consistent with this, they do not increase during light-induced chloroplast development in resting cells and are found to be enriched in the mitochondrial fraction. Further resolution of this fraction on sucrose gradients shows that the APS sulfotransferase is associated with both the microbody (glyoxysomal) and mitochondrial fractions while the thiosulfonate reductase and O-acetyl-L-serine sulfhydrylase are associated only with the mitochondria. Thus the three known enzymes of the bound pathway of assimilatory sulfate reduction are present in Euglena mitochondria.

Although the activity of the entire bound pathway (APS to cysteine) is low in extracts, addition of dithiothreitol which releases free sulfite from the product of the APS sulfotransferase reaction, causes an increase in reduction activity indicating that a sulfite reductase is also present. It remains to be shown which reducing system is the significant one in vivo in Euglena.

³ Abraham and Etta Goodman Professor of Biology. To whom reprint requests should be sent.

¹ Research was supported by Grant BMS73-00987 AO1 from the National Science Foundation.

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