Heterogeneous Nuclear Ribonucleoprotein H1 Coordinates with Phytochrome and the U1 snRNP Complex to Regulate Alternative Splicing in Physcomitrella patens.

Plant photoreceptors tightly regulate gene expression to control photomorphogenic responses. Although gene expression is modulated by photoreceptors at various levels, the regulatory mechanism at the pre-mRNA splicing step remains unclear. Alternative splicing (AS), a widespread mechanism in eukaryotes that generates two or more mRNAs from the same pre-mRNA, is largely controlled by splicing regulators that recruit spliceosomal components to initiate pre-mRNA splicing. The red/far-red light photoreceptor phytochrome participates in light-mediated splicing regulation, but the detailed mechanism remains unclear. By taking protein-protein interaction approaches, we demonstrate that Physcomitrella patens phytochrome 4 (PpPHY4) physically interacts with the heterogeneous nuclear ribonucleoprotein H1 (PphnRNP-H1) splicing regulator in the nucleus, a process dependent on red light. We show that PphnRNP-H1 is involved in red-light-mediated phototropic responses in P. patens, and binds with higher affinity to the pre-mRNA-processing factor 39-1 (PpPRP39-1) splicing factor in the presence of red-light-activated phytochromes. Furthermore, PpPRP39-1 associates with the core component of U1 small nuclear RNP (PpU1C) in P. patens. Genome-wide analyses demonstrated the involvement of both PphnRNP-H1 and PpPRP39-1 in light-mediated splicing regulation. Our results suggest that phytochromes target the early step of spliceosome assembly via a cascade of protein-protein interactions to control pre-mRNA splicing and photomorphogenic responses.