A radiological analysis of the transcription units for heterogeneous nuclear RNA in cultured murine cells.

Ultraviolet irradiation of cultured murine cells damages chromosomal DNA, thus causing premature termination of synthesis of hnRNA chains. We have utilized this effect for measuring the sizes of transcription units for the various classes of hnRNA. Target size analyses of the loss of synthesis of hnRNA species suggest that hnRNA, through its entire size range, constitutes mostly primary transcripts-that is, the shorter RNA molecules are not processed from longer precursors. Polyadenylated hnRNA appears to be a primary transcript as well. Our data, however, are also consistent with processing of precursors to poly(A)+ hnRNA from the 5'termini of larger hnRNA molecules. They are not consistent with models which postulate that poly(A)+ hnRNA sequences originate at the 3' termini of precursors which are considerably larger (greater than 30%) than the polyadenylated hnRNA molecules themselves. Finally, we have found that the process of polyadenylation is not measurably affected by ultraviolet irradiation of cells.