hy8 long hypocotyl mutants of Arabidopsis defective in responsiveness
to prolonged far-red light (the so-called ''far-red high-irradiance re
sponse'') are selectively deficient in functional phytochrome A. To de
fine the molecular lesion in these mutants, we sequenced the phytochro
me A gene (phyA) in lines carrying one or other of two classes of hy8
alleles. The hy8-1 and hy8-2 mutants that express no detectable phytoc
hrome A each have a single nucleotide change that inserts a translatio
nal stop codon in the protein coding sequence. These results establish
that phyA resides at the HY8 locus. The hy8-3 mutant that expresses w
ild-type levels of photochemically active phytochrome A has a glycine-
to-glutamate missense mutation at residue 727 in the C-terminal domain
of the phyA sequence. Quantitative fluence rate response analysis sho
wed that the mutant phytochrome A molecule produced by hy8-3 exhibited
no detectable regulatory activity above that of the phyA-protein-defi
cient hy8-2 mutant. This result indicates that glycine-727, which is i
nvariant in all sequenced phytochromes, has a function important to th
e regulatory activity of phytochrome A but not to photoperception.