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Sulfur Assimilation in C₄ Plants

Intercellular Compartmentation of Adenosine 5'-Triphosphate Sulfurylase in Crabgrass Leaves ¹

^a Biochemistry Department, University of Georgia, Athens, Georgia 30602

The activity of adenosine 5' triphosphate sulfurylase was determined in crabgrass mesophyll cells, bundle sheath strands, and whole leaf extracts. The enzyme was assayed by following molybdate-dependent pyrophosphate release from ATP, ³⁵SO₄^2– incorporation into adenosine 5' phosphosulfate, and ATP synthesis dependent upon adenosine 5' phosphosulfate and inorganic pyrophosphate. With all assays, greater than 90% of the activity was found in extracts from bundle sheath strands. The activities in whole leaf extracts were consistently intermediate between the activities of mesophyll and bundle sheath extracts and extract-mixing experiments gave no indication of enzyme activation or inhibition in vitro. Whole leaf activities were several hundred-fold less than concurrent measurements of ribulose 1,5-bisphosphate and phosphoenolpyruvate carboxylase activities, which is interpreted as being consistent with the relative amounts of elemental carbon and sulfur found in higher plants. A hypothesis is presented for the intercellular compartmentation of sulfur assimilation in relationship to NO₃^– and CO₂ assimilation in leaves of C₄ plants.

¹ This research was supported in part by National Science Foundation Grant PCM 770 8548.

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