Most eukaryotic protein coding nuclear genes are interrupted by non-coding sequences called introns. After gene transcription, introns are excised out of the precursor-mRNA in a process called splicing. This is achieved by a macromolecule called the spliceosome, which is made up of the U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins. Introns are typically identified by the consensus dinucleotides GT and AG at the 5’ and 3’ intron boundaries, respectively.
There are two main splicing mechanisms, constitutive splicing and alternative splicing (AS). AS is a mechanism in which pre-mRNA can form several structurally and functionally different protein products from a single gene. The Serine/Arginine-rich (SR) proteins are a family of conserved essential splicing factors for the intron-recognition, spliceosome assembly and for the regulation of AS. In has been shown in Arabidopsis thaliana that AS of SR-proteins are controlled in developmental and tissue-specific manner, determined by SR-protein quantities.

Physcomitrella patens belongs to the division Bryophyta (mosses) and characteristic to all plants, have an alternation of generations in their life-cycle. The moss has a dominant haploid gametophytic and a diploid sporophytic generation. The gametophytes of P. patens are monoecious, that is, have both male and female gamete-producing organs, called antheridia and archegonia, respectively. Upon gamete fertilization, a diploid zygote is produced, which undergo cell division until a multicellular mature sporophyte emerges. The mature sporophyte produces haploid spores through meiosis that in turn will grow in to a filamentous juvenile gametophyte stage called protonema. Protonema consists of two cell-types, chloronema that have densely packed cells with large chloroplasts and caulonema that have less chloroplasts with less chlorophyll. After spore-germination or protoplast regeneration, chloronemal filaments are produced, from which caulonemal filaments can be branched. Buds originate from caulonema, which later develops into a leafy gametophyte and thus completing the life cycle of P. patens.

Identify and characterize SR-proteins of the SR-subfamily in the moss Physcomitrella patens