Translation affects immunoglobulin mRNA stability

Abstract

When termination codons were introduced into exons of the gene for Ig mu chain, steady-state levels of mu mRNA were reduced, both at the pre-B cell stage and at the plasma cell stage. A termination codon in the variable region gene segment and a termination codon in the second exon of the constant region gene segment had effects of similar magnitude. When the termination codon was deleted, the original level of mRNA was restored. The rate of mu gene transcription was the same whether or not a termination codon was present. Therefore, the termination codons must reduce the amount of the mRNA by reducing its stability. Since the introduced termination codons prematurely terminate translation and, in so doing, change the ribosome load on the mRNA, we conclude that mu mRNA stability is conferred in part by ribosomal protection from enzymatic degradation. We propose that the differences in mu mRNA stability during B lymphocyte differentiation are due to different amounts of ribosomes available for translation.

A Regulatory Mechanism That Detects Premature Nonsense Codons in T-cell Receptor Transcripts in Vivo Is Reversed by Protein Synthesis Inhibitors in Vitro

“…Thus, the most likely mechanisms by which ptcs depress TCR-␤ mature mRNA levels are by: 1) inhibiting the splicing of one or more introns present in TCR-␤ pre-mRNAs or 2) decreasing the stability of partially or fully spliced TCR-␤ transcripts. Our conclusions are consistent with several reports that show that ptcs have no effect on gene transcription in cultured cell lines (8,12,16,19). In contrast, no consensus has emerged regarding the effect of ptcs on pre-mRNA levels in cell lines; ptcs exert no discernible effect on triosephosphate isomerase pre-mRNA levels and thus appear not to affect splicing (19), while ptcs apparently inhibit the splicing of Ig-chain and MVM pre-mRNAs (9,15).…”

“…Consistent with this hypothesis, most Ig and TCR cDNA clones obtained from cDNA libraries have been in-frame (3)(4)(5)(6). Studies with cultured cells have provided evidence that out-of-frame Ig transcripts are downregulated compared with in-frame transcripts (7)(8)(9)(10). In addition, several other genes exhibit depressed mRNA levels when mutated to contain ptcs (Refs.…”

“…1-3). Similarly, it has been shown that endogenous out-of-frame Ig genes are expressed at almost undetectable levels (up to 100-fold less than in-frame genes), while transfected out-of-frame Ig genes are less dramatically down-regulated (7)(8)(9)(10). The presence of ptcs depressed dihydrofolate reductase mRNA levels by 5-20-fold when the mRNA was derived from the endogenous gene, but not at all when the mRNA was derived from stably transfected dihydrofolate reductase genomic clones (12).…”

Endogenous Expression of Activation-Induced Cytidine Deaminase in Cell Line WEHI-231

“…One might argue that transcription dependent activity of AID ought to be similar at the rearranged active and silent alleles. Although in general, due to nonsense-mediated mRNA degradation the levels of mRNA encoding the silent allele are much lower (38), transcription levels are similar to the one of the active allele. It is also possible that mRNA is involved as well in the hypermutation process, which may decrease the rate at the silent allele.…”

Pro-B cells sense productive immunoglobulin heavy chain rearrangement irrespective of polypeptide production

“…Although transcription rates of productively and nonproductively rearranged IgH loci are similar (28), only transcripts from a productively rearranged (coding) allele are stable and accumulate. In contrast, noncoding (nonsense) mRNA from a nonproductively rearranged allele is rapidly degraded by the nonsensemediated mRNA decay (NMD) mechanism (29)(30)(31). Thus, stable coding μH mRNA could indicate the presence of a productive IgH allele and exclude the rearrangement of the other allele, whereas unstable μH mRNA encoded by a nonproductively rearranged IgH gene would be degraded and have no effect.…”