RELATIONSHIP BETWEEN SIZE, STABILITY AND ABUNDANCE OF THE MESSENGER-RNA OF MOUSE L-CELLS

The steady state population of poly(A)+ mRNA from mouse L cells was fractionated into three size classes, <15S, 15S-24S and > 24S, and the nucleotide complexity of each class was determined by kinetic analysis of hybridization to a set of corresponding cDNA probes. A striking difference in complexity was found: the larger mRNA components comprise a diverse class of molecules in which most members were present at relatively low frequencies, while the smaller mRNAs comprise a class of lower complexity in which a greater proportion of the members were present at a relatively high frequency. A comparison of the sedimentation behavior of poly(A)+ mRNA from pulse-labeled cells and from long-term labeled and chased cells indicated that the more stable species of poly(A)+ mRNA were considerably smaller on average than the more rapidly turning over species. Moreover, when differentially labeled stable and unstable mRNAs were simultaneously hybridized with excess cDNA transcribed from steady state poly(A)+ mRNA, we observed that the stable species constitute a major portion of the low complexity class. Thus the correlations between relatively small size, high stability and abundance are all consistent in indicating that differential turnover of cytoplasmic mRNA contributes significantly to the determination of mRNA abundancy in mouse L cells. The results also have important implications for the quantitative analysis of hybridization kinetics of unfractionated mRNA preparations. A bias for large mRNA components in the high complexity class, coupled with the tendency of reverse transcriptase to terminate prematurely while copying large mRNAs, can result in significant distortions of the mRNA frequency distribution. © 1979.