LAF1, a MYB transcription activator for phytochrome A signaling

The photoreceptor phytochrome (phy) A has a well-defined role in regulating gene expression in response to specific light signals. Here, we describe a new Arabidopsis mutant, laf1 (long after far-red light 1) that has an elon gated hypocotyl specifically under far-red light. Gene expression studies s howed that laf1 has reduced responsiveness to continuous far-red light but retains wild-type responses to other light wavelengths. As far-red light is only perceived by phyA, our results suggest that LAF1 is specifically invo lved in phyA signal transduction. Further analyses revealed that laf1 is af fected in a subset of phyA-dependent responses and the phenotype is more se vere at low far-red fluence rates. LAF1 encodes a nuclear protein with stro ng homology with the R2R3-MYB family of DNA-binding proteins. Experiments u sing yeast cells identified a transactivation domain in the C-terminal port ion of the protein. LAF1 is constitutively targeted to the nucleus by signa ls in its N-terminal portion, and the full-length protein accumulates in di stinct nuclear speckles. This accumulation in speckles is abolished by a po int mutation in a lysine residue (K258R), which might serve as a modificati on site by a small ubiquitin-like protein (SUMO).