Antisense HEMA1 RNA expression inhibits heme and chlorophyll biosynthesis in Arabidopsis

5-Aminolevulinic acid (ALA) is a precursor in the biosynthesis of tetrapyrr oles including chlorophylls and heme. The formation of ALA involves two enz ymatic steps which take place in the chloroplast in plants. The first enzym e, glutamyl-tRNA reductase, and the second enzyme, glutamate-1-semialdehyde -2,1-aminomutase, are encoded by the nuclear HEMA and GSA genes, respective ly. To assess the significance of the HEMA gene for chlorophyll and heme sy nthesis, transgenic Arabidopsis plants that expressed antisense HEMA1 mRNA from the constitutive cauliflower mosaic virus 35S promoter were generated. These plants exhibited varying degrees of chlorophyll deficiency, ranging from patchy yellow to total yellow. Analysis indicated that these plants ha d decreased levels of chlorophyll, non-covalently bound hemes, and ALA; the ir levels were proportional to the level of glutamyl-tRNA reductase express ion and were inversely related to the levels of antisense HEMA transcripts. Plants that lacked chlorophyll failed to survive under normal growth condi tions, indicating that HEMA gene expression is essential for growth.