Membrane-bound ribosomes of myeloma cells. IV. mRNA complexity of free and membrane-bound polysomes.

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Abstract
The sequence complexity, frequency distribution and template activity of free (F) and membrane-bound (MB) polysomal mRNA populations of MOPC 21 (P3K) mouse muyeloma cells were analyzed. Using mRNA-c[complementary]DNA hybridization, F poly(A)+ RNA, which represents 60% of total polysomal mRNA, consists of .apprx. 8000 different mRNA sequences distributed in 3 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 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); 25% of F mRNA lacks poly(A). The use of a cDNA highly specific for the Ig L-chain mRNA allows the determination of the subcellular content of this message. Ig L mRNA, representing .apprx. 5% of total polysomal poly(A)+ RNA, is one of the most abundant MB mRNA. Of the Ig L mRNA, 90% is found in MB polysomes and 10% in F polysomes.