ON THE LINKAGE BETWEEN RNA PROCESSING AND RNA TRANSLATABILITY

The immunoglobulin mu heavy chain gene of mouse hybridoma cells is exp ressed in two forms, mu s and mu m, differing in their use of 3' exons . As for many other mammalian genes, mutations in the mu gene which pr ematurely terminate translation often have the effect of reducing the amount of these mu RNAs. To test the generality of this relationship, we selected mutant hybridoma cell lines defective in IgM production an d searched both for translation termination mutations which do not red uce the amount of mu RNA as well as for mutants which show the more co mmonly observed reduction in mu RNA. As observed previously, the amoun t of us RNA is normal in mutants terminating in the C mu 4 exon; by co ntrast the amount of um RNA is reduced in these mutants, indicating th at the effect of the mutation is influenced by some feature near the 3 ' end of the RNA. Mutations terminating translation in other C region exons have a graded effect on RNA content, ranging from 10% the normal level for termination in the C mu 3 exon down to 1% for termination i n the C mu 2 exon. By contrast, a mutant cell line terminating in the leader exon contained 25% the normal amount of mu RNA, suggesting that translation past some point might be required to fully engage the RNA degradation process.