Regulation of 5-Aminolevulinic Acid Biosynthesis In Rhodobacter Sphaeroides 2.4.1

Oakland University, Rochester MI

Investigators

Abstract

As the common precursor to all tetrapyrroles, important biomolecules that include hemes, (bacterio)chlorophyll, and vitamin B12, 5-aminolevulinic acid (ALA) formation is crucial to both energy metabolism and other biosynthetic processes. This project focuses on an investigation of the regulated synthesis of this essential compound in the metabolically versatile bacterium, Rhodobacter sphaeroides 2.4.1. The metabolic flexibility of this organism is made possible by its ability to form all of these tetrapyrroles; aerobic and anaerobic respiratory chains require heme-containing cytochromes, the photosynthetic apparatus requires bacteriochlorophyll, and key biosynthesis enzymes require the cofactor vitamin B12 for functionality. To accommodate the exceedingly variable need in both types and quantities of tetrapyrroles in these cells, ALA formation in R. sphaeroides is responsive to changes in many environmental parameters, including oxygen availability, incident light intensity, carbon source, nitrogen source, and iron availability. This responsiveness in ALA synthesis is due to the highly regulated expression of hemA, which encodes ALA synthase, the enzyme that catalyzes ALA formation in this organism. Thus, the hemA gene constitutes a powerful system for examining how cells integrate complex regulatory networks in R. sphaeroides 2.4.1. Sequence elements within the hemA gene that are necessary for its regulated expression have been identified. Other sequences that are not part of the hemA gene, but which affect its expression, have also been discovered. These sequences will be investigated in more detail to determine how the cell achieves the optimum expression of hemA, such that the total ALA requirement for all the various tetrapyrroles, in all their various concentrations, is met. Because ALA is the necessary ingredient for the formation of molecules that are indispensable to the cell, understanding how its formation is regulated is of broad significance. In R. sphaeroides, as in animal cells, ALA is formed by the enzyme ALA synthase. Thus, R. sphaeroides is a prokaryotic paradigm of this critical biosynthetic reaction. The highly regulated hemA gene, coding for ALA synthase, constitutes an appropriate and amenable model to address questions of how cells can process multiple regulatory signals, since this gene is designed to respond appropriately to changing needs for ALA.