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 viathe action of photoreceptors which are responsive to the red/far-red,blue and UV regions of the spectrum. Phytochrome B (the apoproteinof which is encoded by the PHYB gene) is one of the red/far-redabsorbing photoreceptors active in this process. In this paper,the isolation and characterization of three new EMS-inducedmutations of Arabidopsis which confer phytochrome B deficiencyare described. Complementation analysis showed that these mutations(phyB-101, phyB-102 and phyB-104) were allelic with PHYB. DNAsequence analysis showed that all three mutants contain nucleotidesubstitutions in the PHYB-101 gene sequence. phyB-101 carriesa nucleotide substitution within the second exon of the PHYBgene. This G-to-A substitution is a missense mutation that convertsa glutamate residue at position 812 of the phytochrome B apoproteinto a lysine residue. phyB-102, another missense mutant, carriesa C-to-T substitution which converts a serine residue at position349 of the phytochrome B apoprotein to a phenylalanine residue.phyB-104 carries a premature stop codon as a result of a G-to-Amutation 1190 bp down-stream of the ATG start codon of the PHYBsequence. The missense mutations in phyB-101 and phyB-102 causesignificant alterations in the predicted second ary structureof their respective mutant polypeptides, and identify aminoacid residues playing crucial roles in phytochrome B function,assembly or stability. Key words: Arabidopsis thaliana, phytochromet, phyB mutants, missense mutations