LIGHT REGULATION OF CHLOROPHYLL BIOSYNTHESIS AT THE LEVEL OF 5-AMINOLEVULINATE FORMATION IN ARABIDOPSIS

5-Aminolevulinic acid (ALA) is the universal precursor of tetrapyrrole s, such as chlorophyll and heme. The major control of chlorophyll bios ynthesis is at the step of ALA formation. In the chloroplasts of plant s, as in Escherichia coli, ALA is derived from the glutamate of Glu-tR NA via the two-step C-5 pathway. The first enzyme, Glu-tRNA reductase, catalyzes the reduction of Glu-tRNA to glutamate 1-semialdehyde with the release of intact tRNA. The second enzyme, glutamate 1-semialdehyd e 2,1-aminomutase, converts glutamate 1-semialdehyde to ALA. To furthe r exam ne ALA formation in plants, we isolated Arabidopsis genes that encode the enzymes of the C-5 pathway via functional complementation o f mutations in the corresponding genes of E. coli. The Glu-tRNA reduct ase gene was designated HEMA and the glutamate 1-semialdehyde 2,1-amin omutase gene, GSA1. Each gene contains two short introns (149 and 241 nucleotides for HEMA, 153 and 86 nucleotides for GSA1). The deduced am ino acid sequence of the HEMA protein predicts a protein of 60 kD with substantial similarity (30 to 47% identity) to sequences derived from the known hemA genes from microorganisms that make ALA by the C-5 pat hway. Purified Arabidopsis HEMA protein has Glu-tRNA reductase activit y. The GSA1 gene encodes a 50-kD protein whose deduced amino acid sequ ence shows extensive homology (55 to 78% identity) with glutamate 1-se mialdehyde 2,1-aminomutase proteins from other species. RNA gel blot a nalyses indicated that transcripts for both genes are found in root, l eaf, stem, and flower tissues and that their levels are dramatically e levated by light. Thus, light may regulate ALA, and hence chlorophyll formation, by exerting coordinated transcriptional control over both e nzymes of the C-5 pathway.