Further evidence that the majority of primary nuclear RNA transcripts in mammalian cells do not contribute to mRNA.

Nuclear RNA from Chinese hamster ovary cells was effectively separated into polyadenylic acid [poly(A)]-containing [poly (A)+] and non-poly(A)-containing [poly(A)-] fractions so that -90% of the poly(A) was present in the (A)+ fraction. Only 25% of the 5'-terminal caps of the large nuclear molecules were present in the (A)+ class, but about 70% of the specific mRNA sequences (assayed with cDNA clones) were in the (A)+ class. It appears that many long capped heterogeneous nuclear RNA molecules are of a different sequence category from those molecules that are successfully processed into mRNA.