Light Regulation of Chlorophyll Biosynthesis at the Level of 5-Aminolevulinate Formation in Arabidopsis

Abstract

5-Aminolevulinic acid (ALA) is the universal precursor of tetrapyrroles, such as chlorophyll and heme. The major control of chlorophyll biosynthesis is at the step of ALA formation. In the chloroplasts of plants, as in Escherichia coli, ALA is derived from the glutamate of Glu-tRNA via the two-step C5 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-semialdehyde 2,1-aminomutase, converts glutamate 1-semialdehyde to ALA. To further examine ALA formation in plants, we isolated Arabidopsis genes that encode the enzymes of the C5 pathway via functional complementation of mutations in the corresponding genes of E. coli. The Glu-tRNA reductase gene was designated HEMA and the glutamate 1-semialdehyde 2,1-aminomutase gene, GSA1. Each gene contains two short introns (149 and 241 nucleotides for HEMA, 153 and 86 nucleotides for GSA1). The deduced amino 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 C5 pathway. Purified Arabidopsis HEMA protein has Glu-tRNA reductase activity. The GSA1 gene encodes a 50-kD protein whose deduced amino acid sequence shows extensive homology (55 to 78% identity) with glutamate 1-semialdehyde 2,1-aminomutase proteins from other species. RNA gel blot analyses indicated that transcripts for both genes are found in root, leaf, stem, and flower tissues and that their levels are dramatically elevated by light. Thus, light may regulate ALA, and hence chlorophyll formation, by exerting coordinated transcriptional control over both enzymes of the C5 pathway.

The Regulation of Enzymes Involved in Chlorophyll Biosynthesis

“…Their amino acid sequences display a much higher evolutionary conservation than expected from the enzyme parameters, however (Pontoppidan and Ilag et al, 1994). Gluramyl-tRNA reductase is likely to be a nuclear-encoded plastid protein (Ilag et al, 1994).…”

“…GlutamyltRNA reductases purified from different sources exhibit a significant variability in their catalytic activities, subunit compositions, and molecular masses Pontoppidan and Kannangara, 1994). Their amino acid sequences display a much higher evolutionary conservation than expected from the enzyme parameters, however (Pontoppidan and Ilag et al, 1994). Gluramyl-tRNA reductase is likely to be a nuclear-encoded plastid protein (Ilag et al, 1994).…”

“…Their encoded products seem to be very similar to one another and appear to display also a high degree of sequence similarity to the hemL gene product of E. coli and other bacteria, which synthesize 5-aminolevulinic acid via the C, pathway (Grimm, 1990;Sangwan and O'Brian, 1993;Ilag et al, 1994;Matters and Beale, 1994). Plant glutamate-1-semialdehyde aminotransferase is a nuclear gene product (Grimm, 1990;Sangwan and O'Brian, 1993;Ilag et al, 1994).…”

“…A few glutamate-1-semialdehyde aminotransferase cDNAs have been isolated (Grimm, 1990;Hofgen et al, 1994;Ilag et al, 1994;Matters and Beale, 1994). Their encoded products seem to be very similar to one another and appear to display also a high degree of sequence similarity to the hemL gene product of E. coli and other bacteria, which synthesize 5-aminolevulinic acid via the C, pathway (Grimm, 1990;Sangwan and O'Brian, 1993;Ilag et al, 1994;Matters and Beale, 1994).…”

“…For example, glutamyl-tRNAG'" of Escherichia coli could serve as substrate of the enzyme from Arabidopsis thaliana (Ilag et al, 1994) but was not recognized by the purified barley protein (Pontoppidan and . Barley glutamyl-tRNA reductase also did not use cytosolic glutamyl-tRNAG'" in a homologous in vitro system (Peterson et al, 1988), which demonstrates its high tRNA specificity.…”