Molecular Charactrization of a phyA Mutant impaired in the HIR response

MATEOS,JULIETA LISA; LUPPI, JUAN PABLO; YANOVSKY, MARCELO; GAERTNER, WOLFGANG; CASAL, JORGE; BRASLAVSKY, SILVIA

Marburg, Germany

Congreso; Plant Photobiology Meeting 2003; 2003

Phytochromes are a family of photoreceptors able to sense the light environment(1) This property of the protein derives from  its ability to photoconvert between red (Pr) and far-red (Pfr) light absorbing forms. Among  the five phytochrome members (phyA – E) that exist in Arabidopsis, phyA is the most abundant in dark-grown seedlings. Even when a single physiological process is considered (e.g. hypocotyls growth inhibition), two components of phytochrome A action can be distinguished: very-low-fluence response (VLFR), which is saturated at low fluences of R or FR; and the high-irradiance response (HIR), which requires sustained light activation(2). The aim of this work is to investigate whether the action of phyA in HIR involves specific photochemical properties of the photoreceptor. An Arabidopsis mutant impaired in the HIR, but not in the VLFR component has been isolated. This mutant is allelic to the phyA-201 null mutant and the sequencing results show a nucleotide change in the N-terminal (Nt) domain. Physiological measurements indicate that this mutation (phyA-303) severely affects the fluence rate dependence of the phyA action under FR. Attempts to express the Nt-domain of the mutant protein via heterologous expression in yeast have not been successful so far. However, site-directed mutagenesis has been performed to incorporate the phyA-303 mutation  in the background of the PHYA gene of Avena  in order to investigate the effects on the photochemical properties of the photoreceptor. Preliminary results show that the difference spectra of both molecules (oat wild type and mutant) exhibit the same maxima and minima when assembly with PCB. However, chromophore incorporation appears to be slower in the mutant.