Sulfate activation (beginning with ATP sulfurylase), along with sulfate reduction to cysteine (beginning with APS sulfotransferase), is located on the inner mitochondrial membrane of Euglena, but is absent from the chloroplasts. Sulfite (or cysteine) from the mitochondria is used by the chloroplasts as a precursor of the sulfolipid sulfonic acid group. This research examines 1) phosphate inhibition of purified mitochondrial ATP sulfurylase as a means of determining its regulatory significance, 2) the cellular location and properties of a second different ATP sulfurylase purified from whole cells and, 3) the mechanism of purified APS sulfotransferase. The flow of sulfur compounds out of the mitochondrion to chloroplast serves as a model system in which to explore the origin of chloroplast constituents during plastid development and the nature of mitochondrial transport processes. Sulfate is the common form of sulfur in aerobic environments. Sulfate must be metabolically activated before it can be used for important reactions in cells. Since sulfur is an essential nutrient for plants, an understanding of sulfate metabolism is essential for understanding the normal agricultural (and ecological) utilization of sulfur, as well as the consequences of sulfur deficiency and its correction. This research extends the examiniation of sulfur metabolism to the chloroplast and mitochondrion of the unicellular alga, Euglena because recent evidence indicates that there is a flow of sulfur metabolites between these two organelles. Therefore, this research provides not only for an understanding of sulfate metabolism at the subcellular level, but also shows how these organelles are coordinately controlled and how they share the cellular resources available for organelle development.
