These same U5 snRNA mutations also interact genetically with certain U2 snRNA mutations that lie in the helix I and helix II regions of the U2-U6 snRNA structure. Mutations in loop 1 of U5 snRNA, a region that is implicated in the alignment of the two exons, are synthetically lethal with slu4/prp17-2 and slu7-1 (D. Pairwise synthetic lethality was observed among slt11-1 (which affects the first step of splicing) and several second-step factors, including slt15/prp17-100, slt17/slu7-100, and prp16-1. This difference suggests that the latter two factors are unlikely to be involved in interactions with U2-U6 snRNA helix II but rather are specific to interactions with U2 snRNA. slt11-1 and slt22-1 are synthetically lethal with mutations in the 3′ end of U6 snRNA, a region that affects U2-U6 snRNA helix II however, slt17/slu7-100 and slt21/prp8-21 are not. The remaining four slt mutations are new alleles of previously identified splicing genes: slt15, previously identified as prp17 ( slt15/prp17-100), slt16/smd3-1, slt17/slu7-100, and slt21/prp8-21. SLT11 encodes a new splicing factor and SLT22 encodes a new RNA-dependent ATPase RNA helicase (D.

Six slt (stands for synthetic lethality with U2) mutants were isolated on the basis of synthetic lethality with a U2 snRNA mutation that perturbs the U2-U6 snRNA helix II interaction. A genetic screen was devised to identify Saccharomyces cerevisiae splicing factors that are important for the function of the 5′ end of U2 snRNA.

State of survival lethality vs attack