Nucleotide sequence of an Arabidopsis cDNA for phytoene synthase.

Carotenoid biosynthesis is a process vital to a11 photosynthetic organisms. Carotenoids provide photoprotection for the photosynthetic apparatus by dissipating excess energy in the form of heat (Koyama, 1991). PSY catalyzes the formation of phytoene, the first dedicated intermediate in the carotenoid biosynthetic pathway. We have previously isolated two PSY cDNAs from tomato that correspond to two PSY genes (Bartley et al., 1992; Bartley and Scolnik, 1993). To extend our study of carotenoid biosynthesis to Arabidopsis thaliana, we have isolated and sequenced a cDNA for PSY from this organism. A XZAP library constructed from etiolated seedlings was screened by hybridization with a 32Plabeled probe generated by the PCR from the PSYl cDNA of tomato. As in the A. thaliana phytoene desaturase cDNAs, this cDNA contains no poly(A) tail, possibly due to exonuclease activity during the construction of the library (Scolnik and Bartley, 1993). Sequence analysis of the cDNA revealed an open reading frame encoding a protein of 423 amino acid residues (Table I). Alignment of the amino acid sequence with PSY‘s from Erzuinia uredovora ( c r fB) (Misawa et al., 1990) and Rhodobacter capsulafus (crtB) (Armstrong et al., 1989) suggests that the protein contains a putative transit peptide of 150 amino acid residues at the N terminus and a mature peptide of 273 amino acids. Comparison of the mature peptide sequence with the mature peptides of PSYl and PSY2 of tomato shows approximately 92% similarity and approximately 85% identity in both cases. Transit peptide similarity and identity of the A. thaliana PSY with PSYl and PSY2 of tomato indicates some conservation at the amino acid leve1 (Bartley et al., 1992; Ray et al., 1992).