A 2ND-STEP SPLICING ACTIVITY IS CONSERVED FROM YEAST TO HUMAN

We describe a defective HeLa nuclear extract which is particularly def icient in step 2 of splicing reaction. With this extract we have studi ed the conservation of a second-step activity from yeast to human cell s. We detected a S. cerevisiae second step splicing activity that allo ws restoration of step 2 of the defective HeLa nuclear extract, which indicates that yeast purified fraction has a second-step activity that is conserved from yeast to human cells. The activity is a yeast UsnRN P protein(s) since it is purified with anti-trimethylguanosine by immu noaffinity columns. (C) 1998 Academic Press.