Base composition of fractions of nuclear and nucleolar ribonucleic acid obtained by sedimentation and chromatography.

Sufficient quantities of various nuclear and nucleolar ribonucleic acid fractions were obtained by repeated density gradient sedimentations for analysis of base composition determined by optical density, as well as base composition determined by labeling with 32P-orthophosphate, reaction with formaldehyde, and chromatography on methylated serum albumin columns. The base compositions determined by optical density of 45, 35, and 28 S nucleolar ribonucleic acid fractions were identical within the limits of experimental error. A similar result was obtained for base composition, determined by labeling with 32P-orthophosphate, of these fractions each of which had a ratio of adenylic plus uridylic acid to guanylic plus cytidylic acid (A + U:G + C) of approximately 0.6. These results support the conclusions of previous studies that 28 S ribonucleic acid is a product of 45 S ribonucleic acid synthesized in the nucleolus. Evidence that the hydrogen bonding of these fractions differed was obtained by studies with formaldehyde which suggest that 90% of the amino groups in 35 and 45 S ribonucleic acid are hydrogen-bonded as compared to 75% in the 28 S ribonucleic acid in the nucleolar ribonucleic acid fractions. Analyses of the extranucleolar nuclear ribonucleic acid and total nuclear ribonucleic acid indicate that a 45 S ribonucleic acid is formed in the extranucleolar ribonucleic acid which has a ratio of adenylic plus uridylic acid to guanylic plus cytidylic acid (A + U:G + C) of 1.16 by 32P base analysis. It seems possible that this ribonucleic acid may be the precursor of the 18 S ribonucleic acid and messenger ribonucleic acid which is rich in adenine and uridine.