IDENTIFICATION AND CHARACTERIZATION OF A YEAST GENE ENCODING THE U2 SMALL NUCLEAR RIBONUCLEOPROTEIN PARTICLE B'' PROTEIN

The inessential yeast gene MUD2 encodes a protein factor that contribu tes to U1 small nuclear ribonucleoprotein particle (snRNP)-pre-mRNA co mplex (commitment complex) formation. To identify other genes that con tribute to this early splicing step, we performed a synthetic lethal s creen with a MUD2 deletion strain. The first characterized gene hom th is screen, MSL1 (MUD synthetic lethal 1), encodes the yeast homolog of the well studied mammalian snRNP protein U2B''. The yeast protein (YU 2B'') is a component of yeast U2 snRNP, and it is related to other mem bers of the U1A-U2B'' family, the human U2B'' protein, the human U1A p rotein, and the yeast U1A protein. It binds in vitro to its RNA target , U2 snRNA stem-loop IV, without a protein cofactor, and the target re sembles more closely the U1 snRNA binding site of the human U1A protei n than it does the U2 snRNA binding site of human U2B''. Surprisingly, the YU2B'' protein lacks a C-terminal RNA binding domain, which is co nserved in all other family members. Possible functional and evolution ary relationships among these proteins are discussed.