The relationship between hnRNA+-poly(A) and mRNA+-poly (A) in non-dividing human lymphocytes. Evidence for distinct synthetic pathways for mRNA precursor- and nonprecursor-hnRNA.

The processing of hnRNA+-poly(A) to mRNA+-poly(A) has been studied in resting lymphocytes from human peripheral blood. In pulse-chase experiments, two types of hnRNA+-poly(A) have been distinguished: the first is labeled predominantly with exogenous radioactive precursors supplied during the pulse, and the second incorporates primarily scavenged labeled precursors made available during a chase incubation. When the disappearance of both types of hnRNA+-poly(A) was quantitatively compared with the appearance of stable and labile mRNA+-poly(A), only 10% of the anticipated cytoplasmic material was actually obtained. Statistically, 90% of the poly(A)-bearing hnRNA molecules processed were degraded. The two types of hnRNA+-poly(A) were found to be functionally different. Pulse-labeled material was processed to poly(A)-bearing mRNA; "chase-labeled" molecules did not leave the nucleus and never served as precursors for cytoplasmic mRNA. The data fit a model in which there are distinct pathways for mRNA precursor- and nonprecursor-hnRNA+-poly(A).