A 2ND AND DIFFERENTIALLY EXPRESSED GLUTAMYL-TRANSFER-RNA REDUCTASE GENE FROM ARABIDOPSIS-THALIANA

5-Aminolevulinic acid (ALA) is the universal precursor of tetrapyrrole s (e.g., chlorophylls and hemes). In the chloroplasts of plants and in several eubacterial species ALA is formed in a two-step process known as the C-5 pathway. In the first step, glutamyl-tRNA reductase (GluTR ), converts glutamate of glutamyl-tRNA to glutamate l-semialdehyde (GS A) which is rearranged to ALA by glutamate 1-semialdehyde-2,1-aminomut ase (GSA-AM) in the second step. Since ALA formation is a limiting ste p in chlorophyll biosynthesis, GluTR, which is encoded by the HEMA gen e in Arabidopsis thaliana plays a vital role in that biosynthesis. Her e we report the occurrence of a second functional HEMA gene (HEMA2) in A. thaliana. This gene was isolated by screening a genomic library wi th a probe from HEMA1. The nucleotide sequence of the cDNA and the cor responding genomic DNA indicates that the Arabidopsis HEMA2 gene conta ins two short introns (285 bp and 159 bp). The deduced amino acid sequ ence predicts a HEMA2 protein of 530 amino acids with 79% identity to the HEMA1-encoded GluTR. The 5'-flanking sequence of the HEMA2 gene in cludes several motifs (e.g., GT-1 boxes, GATA motifs) similar to light -responsive regulatory elements found in light-inducible genes. Unlike the HEMA I transcript, which is present in all parts of the plant, HE MA2 is expressed in low levels in roots and flowers. The presence of a second functional HEMA gene in Arabidopsis raises the possibility tha t two C-5 pathways exist in chloroplasts.