Incubation of in vitro synthesized mouse histone H4 mRNA precursors in nuclear extracts of mouse 3T6 fibroblasts, rat L6-5 myoblasts and myotubes yields processed mRNA species. A processing activity was identified in all three kinds of extracts that cleaves the precursor transcripts, generating mRNA species with mature 3' termini. 2. The processing activity is present both in proliferating (mouse 3T6 fibroblast, rat L6-5 myoblast) and terminally differentiated (rat L6 myotube) cells. 3. The efficiency of the endonucleolytic cleavage reactions was higher in a homologous system, i.e. in the presence of nuclear extracts from mouse 3T6 fibroblasts, than in a heterologous system, i.e. in the presence of rat myoblast or myotube extracts. 4. The in vitro processing activity is specifically inhibited by anti-Sm antibodies, which suggests the requirement of an snRNP particle for H4 pre-mRNA maturation.
