Addition of poly(A) to nuclear RNA occurs soon after RNA synthesis.

Abstract
A kinetic analysis of the appearance of [3H]uridine label in RNA sequences that neighbor poly(A), and the incorporation of [3H]adenosine label into the RNA chain and the poly(A) of poly(A)-containing molecules, shows that poly(A) is added within a minute or so after RNA chain synthesis in Chinese hamster ovary cells and HeLa [human cervical carcinoma] cells. Previous conclusions by several groups that poly(A) might be added as long as 20-30 min after RNA synthesis appear to be in error. Present results seem to be much more in line with several recent studies with specific mRNA that suggest prompt poly(A) addition.