Nucleotide sequence and expression of the phytochrome gene in Pisum sativum: Differential regulation by light of multiple transcripts.

Complementary DNA and genomic DNA that code for phytochrome apoprotein in Pisum sativum cv. Alaska were cloned in λgt11 and λEMBL3, respectively, and sequenced. Southern and slot-blot analysis of pea nuclear DNA showed that there was only one copy of phytochrome gene (phy) per haploid genome. The phy gene, which consisted of five exons and four short introns, coded for a polypeptide comprising 1124 amino acid residues. Detailed analysis of the 5' end of phy transcripts by nuclease S1 mapping and primer extension demonstrated the presence of three distinct transcripts (RNAs 1,2, and 3) differing in the length of the 5' non-coding sequence (63, 285, and about 465 nucleotides long, respectively). A short upstream open reading frame was found in RNA 2, while RNA 3 contained an additional upstream open reading frame. The relative levels of RNAs 1, 2, and 3 per unit weight of poly(A)(+) RNA, which were semi-quantitatively estimated by nuclease S1 mapping, primer extension, and Northern hybridization, were 88, 11, and 1 in the dark, and 1, 6, and 6 in the light, respectively. Shifting of the plants from the dark to the light confirmed that the levels of the three transcripts were regulated by light in different ways, namely, elevated (RNA 3), weakly reduced (RNA 2), and strongly reduced (RNA 1).