Deadenylation of the unstable mRNA encoded by the yeast MFA2 gene leads to decapping followed by 5'-->3' digestion of the transcript

The first step in the decay of some eukaryotic mRNAs is the shortening of the poly(A) tail. To examine how the transcript body was degraded after deadenylation, we followed the decay of a pulse of newly synthesized MFA2 transcripts while utilizing two strategies to trap intermediates in the degradation pathway. First, we inserted strong RNA secondary structures, which can slow exonucleolytic digestion and thereby trap decay intermediates, into the MFA2 5' UTR. Following deadenylation, fragments of the MFA2 mRNA trimmed from the 5' end to the site of secondary structure accumulated as full-length mRNA levels decreased. In addition, in cells deleted for the XRN1 gene, which encodes a major 5' to 3' exonuclease in yeast, the MFA2 transcript is deadenylated normally but persists as a full-length mRNA lacking the 5' cap structure. These results define a mRNA decay pathway in which deadenylation leads to decapping of the mRNA followed by 5'-->3' exonucleolytic degradation of the transcript body. Because the poly(A) tail and the cap structure are found on essentially all mRNAs, this pathway could be a general mechanism for the decay of many eukaryotic transcripts.

A rapid method for localized mutagenesis of yeast genes

We have developed a simple procedure for the localized mutagenesis of yeast genes. In this technique the region of interest is first amplified under mutagenic polymerase chain reaction (PCR) conditions. Cotransformation of the PCR product with a gapped plasmid containing homology to both ends of the PCR product allows in vivo recombination to repair the gap with the mutagenized DNA. This procedure is efficient, allows targeting of specific regions for mutagenesis, and requires no subcloning steps in Escherichia coli.

Premature translational termination triggers mRNA decapping

The degradation of messenger RNA in eukaryotic cells is initiated by endonucleolytic cleavage or by shortening of the poly(A) tail, which for some mRNAs activates a deadenylation-dependent decapping reaction. One type of rapid mRNA degradation in eukaryotes is caused by premature termination of translation. This turnover process prevents the translation of aberrant mRNAs, may affect the abundance and splicing pattern of nuclear transcripts, and may be involved in the aetiology of human genetic disease. Here we show that premature translational termination in yeast triggers decapping, independent of deadenylation, thereby exposing the transcript to 5'-to-3' degradation. Inactivation of the 5'-to-3' exonuclease reveals an additional 3'-to-5' pathway of mRNA turnover. These observations provide in vivo evidence for two new mechanisms of mRNA decay.

Turnover mechanisms of the stable yeast PGK1 mRNA

The first step in the decay of several yeast mRNAs is the shortening of the poly(A) tail, which for the MFA2 transcript triggers decapping and 5'-to-3' degradation. To understand the basis for differences in mRNA decay rates, it is important to determine if deadenylation-dependent decapping is specific to the unstable MFA2 transcript or is a general mechanism of mRNA degradation. To this end, we analyzed the turnover of the stable PGK1 mRNA by monitoring the decay of a pulse of newly synthesized transcripts while using two strategies to trap decay intermediates. First, we used strains deleted for the XRN1 gene, which encodes a major 5'-to-3' exonuclease in Saccharomyces cerevisiae. In xrn1 delta cells, PGK1 transcripts lacking the 5' cap structure and a few nucleotides at the 5' end were detected after deadenylation. Second, we inserted into the PGK1 5' untranslated region strong RNA secondary structures, which can slow exonucleolytic digestion and thereby trap decay intermediates. These secondary structures led to the accumulation of PGK1 mRNA fragments, following deadenylation, trimmed from the 5' end to the site of the secondary structure. The insertion of strong secondary structures into the 5' untranslated region also inhibited translation of the mRNA and greatly stimulated the decay of the PGK1 transcripts, suggesting that translation of the PGK1 mRNA is required for its normally slow rate of decay. These results suggest that one mechanism of degradation of the PGK1 transcript is deadenylation followed by decapping and subsequent 5'-to-3' exonucleolytic degradation. In addition, by blocking the 5'-to-3' degradation process, we observed PGK1 mRNA fragments that are consistent with a 3'-to-5' pathway of mRNA turnover that is slightly slower than the decapping/5'-to-3' decay pathway. These observations indicate that there are multiple mechanisms by which an individual transcript can be degraded following deadenylation.

Mutations affecting stability and deadenylation of the yeast MFA2 transcript

Decay rates of individual mRNAs in the yeast Saccharomyces cerevisiae can vary by 10- to 20-fold. To determine the basis for the rapid degradation of the mRNA encoded by the yeast MFA2 gene we have used a genetic screen to isolate mutations that increase the stability of this transcript. Analysis of point mutations obtained from this screen, and of additional lesions constructed in vitro, indicated that the MFA2 3'-untranslated region (UTR) contains sequences that specify rapid mRNA decay. Moreover, the lesions that affected mRNA decay rate also affected the process of mRNA deadenylation. Mutations in one region of the 3' UTR both decreased the rate of poly(A) shortening and increased the stability of an intermediate form in the decay pathway with an oligo(A) tail (approximately 10 nucleotides). Mutations in a second region primarily increased the stability of the oligo(A) form. These results suggest that the decay of the MFA2 mRNA initiates with the shortening of the poly(A) tail and there are specific sequences within the 3' UTR that stimulate poly(A) tail shortening as well as subsequent steps in the decay pathway. Given the similarity of this decay pathway to that seen for some mammalian mRNAs, these results suggest that mRNA deadenylation may be a common mechanism of mRNA turnover.

Aberrant mRNAs with extended 3' UTRs are substrates for rapid degradation by mRNA surveillance

The mRNA surveillance system is known to rapidly degrade aberrant mRNAs that contain premature termination codons in a process referred to as nonsense-mediated decay. A second class of aberrant mRNAs are those wherein the 3' UTR is abnormally extended due to a mutation in the polyadenylation site. We provide several observations that these abnormally 3'-extended mRNAs are degraded by the same machinery that degrades mRNAs with premature nonsense codons. First, the decay of the 3'-extended mRNAs is dependent on the same decapping enzyme and 5'-to-3' exonuclease. Second, the decay is also dependent on the proteins encoded by the UPF1, UPF2, and UPF3 genes, which are known to be specifically required for the rapid decay of mRNAs containing nonsense codons. Third, the ability of an extended 3' UTR to trigger decay is prevented by stabilizing sequences within the PGK1 coding region that are known to protect mRNAs from the rapid decay induced by premature nonsense codons. These results indicate that the mRNA surveillance system plays a role in degrading abnormally extended 3' UTRs. Based on these results, we propose a model in which the mRNA surveillance machinery degrades aberrant mRNAs due to the absence of the proper spatial arrangement of the translation-termination codon with respect to the 3' UTR element as defined by the utilization of a polyadenylation site.

A small segment of the MAT alpha 1 transcript promotes mRNA decay in Saccharomyces cerevisiae: a stimulatory role for rare codons

Differences in decay rates of eukaryotic transcripts can be determined by discrete sequence elements within mRNAs. Through the analysis of chimeric transcripts and internal deletions, we have identified a 65-nucleotide segment of the MAT alpha 1 mRNA coding region, termed the MAT alpha 1 instability element, that is sufficient to confer instability to a stable PGK1 reporter transcript and that accelerates turnover of the unstable MAT alpha 1 mRNA. This 65-nucleotide element is composed of two parts, one located within the 5' 33 nucleotides and the second located in the 3' 32 nucleotides. The first part, which can be functionally replaced by sequences containing rare codons, is unable to promote rapid decay by itself but can enhance the action of the 3' 32 nucleotides (positions 234 to 266 in the MAT alpha 1 mRNA) in accelerating turnover. A second portion of the MAT alpha 1 mRNA (nucleotides 265 to 290) is also sufficient to destabilize the PGK1 reporter transcript when positioned 3' of rare codons, suggesting that the 3' half of the MAT alpha 1 instability element is functionally reiterated within the MAT alpha 1 mRNA. The observation that rare codons are part of the 65-nucleotide MAT alpha 1 instability element suggests possible mechanisms through which translation and mRNA decay may be linked.

Recognition of yeast mRNAs as "nonsense containing" leads to both inhibition of mRNA translation and mRNA degradation: implications for the control of mRNA decapping

A critical step in the degradation of many eukaryotic mRNAs is a decapping reaction that exposes the transcript to 5' to 3' exonucleolytic degradation. The dual role of the cap structure as a target of mRNA degradation and as the site of assembly of translation initiation factors has led to the hypothesis that the rate of decapping would be specified by the status of the cap binding complex. This model makes the prediction that signals that promote mRNA decapping should also alter translation. To test this hypothesis, we examined the decapping triggered by premature termination codons to determine whether there is a down-regulation of translation when mRNAs were recognized as "nonsense containing." We constructed an mRNA containing a premature stop codon in which we could measure the levels of both the mRNA and the polypeptide encoded upstream of the premature stop codon. Using this system, we analyzed the effects of premature stop codons on the levels of protein being produced per mRNA. In addition, by using alterations either in cis or in trans that inactivate different steps in the recognition and degradation of nonsense-containing mRNAs, we demonstrated that the recognition of a nonsense codon led to a decrease in the translational efficiency of the mRNA. These observations argue that the signal from a premature termination codon impinges on the translation machinery and suggest that decapping is a consequence of the change in translational status of the mRNA.

Analysis of the yeast genome: identification of new non-coding and small ORF-containing RNAs

The genome sequences from increasing numbers of organisms allow for rapid and organized examination of gene expression. Yet current computational-based paradigms for gene recognition are limited and likely to miss genes expressing non-coding RNAs or mRNAs with small open reading frames (ORFs). We have utilized two strategies to determine if there are additional transcripts in the yeast Saccharomyces cerevisiae that were not identified in previous analyses of the genome. In one approach, we identified strong consensus polymerase III promoters based on sequence, and determined experimentally if these promoters drive the expression of an RNA polymerase III transcript. This approach led to the identification of a new, non-essential 170 nt non-coding RNA. An alternative strategy analyzed RNA expression from large sequence gaps>2 kb between predicted ORFs. Fifteen unique RNA transcripts ranging in size from 161 to 1200 nt were identified from a total of 59 sequence gaps. Several of these RNAs contain unusually small potential ORFs, while one is clearly non-coding and appears to be a small nucleolar RNA. These results suggest that there are likely to be additional previously unidentified non-coding RNAs in yeast, and that new paradigms for gene recognition will be required to identify all expressed genes from an organism.

Analysis of chimeric mRNAs derived from the STE3 mRNA identifies multiple regions within yeast mRNAs that modulate mRNA decay

In the yeast Saccharomyces cerevisiae unstable mRNAs decay 10-20 fold more rapidly than stable mRNAs. In order to examine the basis for the differences in decay rate of the unstable STE3 mRNA and the stable PGK1 and ACT1 mRNAs we have constructed and measured the decay rates of numerous chimeric mRNAs. These experiments indicate that multiple regions within yeast mRNAs are involved in modulating mRNA decay rates. Our results suggest that at least two regions within the STE3 mRNA are involved in stimulating rapid decay. One region is located within the coding region and requires sequences between codons 13 and 179. In addition, the STE3 3' UT can also function to stimulate decay. Surprisingly, the STE3 3' UT is not sufficient to accelerate the turnover of the stable PGK1 transcript unless portions of the PGK1 coding region are first deleted. These results not only identify sequences that function within yeast to stimulate mRNA turnover but also have important implications for an understanding of the basis of differences in eukaryotic mRNA decay rates.