Comparison of complexity and diversity of polyadenylated polysomal and informosomal messenger ribonucleic acid from Chinese hamster cells

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Abstract
The sequence complexity and relative abundance of cytoplasmic polyadenylated polysomal (ribosome-bound) mRNA and cytoplasmic polyadenylated informosomal (ribosome-free) mRNA were analyzed in exponentially growing Chinese hamster cells (line CHO) using the technique of c[complementary]DNA hybridization to excess poly(A)+ mRNA. Polysomal and informosomal mRNA had similar complexities (.apprx. 8300 mRNA species), but both the fraction of mRNA and the number of sequences comprising the mRNA abundance classes were different. Heterologous annealing reactions showed that all of the mRNA sequences detected were shared by the polysomal and informosomal mRNA. The most abundant informosomal mRNA component was considerably different from the most abundant polysomal mRNA component. For a more detailed analysis, cDNA complementary to the most abundant informosomal and polysomal mRNA was isolated. By use of the fractionated cDNA, it could be demonstrated that the most abundant informosomal mRNA sequences were distributed in the polysomal mRNA with an approximately 5-fold reduction in relative frequency. These results are not compatible with models postulating translational control of gene expression by the complete sequestering of some mRNA sequences in an untranslatable form in the cytoplasm. The data are consistent with models encompassing differential rates of initiation on the polysome and/or preferential affinity of some mRNA for initiation factors.