We have analyzed the sequence complexity, frequency distribution, and template activity of free (F) and membrane-bound (MB) polysomal mRNA populations of MOPC 21 (P3K) mouse myeloma cells. Using the technique of mRNA-cDNA hybridization, we find that F poly(A)+ RNA, which represent 60% of total polysomal mRNA, consists of approximately 8,000 different mRNA sequences distributed in three abundance classes, while MB poly(A)+ RNA (20% of total polysomal mRNA) includes only 230 mRNA species and almost completely lacks very infrequent mRNA species. Cross-hybridization indicates that MB mRNA sequences are also present in F mRNA, but in reduced concentrations. Translation of F and MB RNA fractions in a messenger-dependent reticulocyte lysate indicates that essentially all MB RNA contains poly(A), whereas 25% of F mRNA lacks poly(A). Furthermore, the use of a cDNA highly specific for the immunoglobulin light (Ig L) chain mRNA allows the determination of the subcellular content of this message. Ig L mRNA, representing approximately 5% of total polysomal poly(A)+ RNA, is one of the most abundant MB mRNAs. 90% of Ig L mRNA is found in MB polysomes and 10% in F polysomes.

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