Identification of phytochrome B amino acid residues mutated in three new phyB mutants of Arabidopsis thaliana

The growth and development of plants is regulated by light via the action of photoreceptors which are responsive to the red/far-red, blue and UV regions of the spectrum. Phytochrome B (the apoprotein of which is encoded by the PHYB gene) is one of the red/far-red absorbing photoreceptors active in this process. In this paper, the isolation and characterization of three new EMS-induced mutations of Arabidopsis which confer phytochrome B deficiency are described. Complementation analysis showed that these mutations (phyB- 101, phyB-102 and phyB-104) were allelic with PHYB. DNA sequence analysis showed that all three mutants contain nucleotide substitutions in the PHYB gene sequence. phyB-101 carries a nucleotide substitution within the second exon of the PHYB gene. This G-to-A substitution is a missense mutation that converts a glutamate residue at position 812 of the phytochrome B apoprotein to a lysine residue. phyB-102, another missense mutant, carries a C-to-T substitution which converts a serine residue at position 349 of the phytochrome B apoprotein to a phenylalanine residue. phyB-104 carries a premature stop codon as a result of a G-to-A mutation 1190 bp downstream of the ATG start codon of the PHYB sequence. The missense mutations in phyB-101 and phyB-102 cause significant alterations in the predicted secondary structure of their respective mutant polypeptides, and identify amino acid residues playing crucial roles in phytochrome B function, assembly or stability.