Role of the transcription activator Ste12p as a repressor of PRY3 expression.

Mating pheromone represses synthesis of full-length PRY3 mRNA, and a new transcript appears simultaneously with its 5' terminus 452 nucleotides inside the open reading frame (ORF). Synthesis of this shorter transcript results from activation of a promoter within the PRY3 locus, and its production is concomitant with the rapid disappearance of the full-length transcript. Evidence is consistent with the pheromone-induced transcription factor Ste12p binding two pheromone response elements within the PRY3 promoter, directly impeding transcription of the full-length mRNA while simultaneously inducing initiation of the short transcript. This process depends on a TATA box within the PRY3 ORF. Expression of full-length PRY3 inhibited mating, while no disadvantage was detectable for cells unable to make the short transcript. Therefore, Ste12p is utilized as a repressor of full-length PRY3 transcription, ensuring efficient mating. There is no evidence that production of the short PRY3 transcript is anything more than an adventitious by-product of this mechanism. It is possible that cryptic binding sites for transcriptional activators may occur frequently within genomes and have the potential of evolving for rapid, gene-specific repression by mechanisms analogous to PRY3. PRY3 regulation provides a model for the coordination of both inductive and repressive activities within a regulatory network.

Silencing the transcriptome's dark matter: mechanisms for suppressing translation of intergenic transcripts.

Large portions of the genomes of higher eukaryotes are transcribed into RNA molecules that are never destined for translation into proteins. Although some of these transcripts have clearly defined biological roles other than protein coding, most arise from genomic regions devoid of functional genes and many are antisense to regions containing annotated genes. A variety of mechanisms exist to prevent adventitious production of proteins from these transcripts, ranging from degradation within the nucleus to translational silencing in the cytosol.

The undertranslated transcriptome reveals widespread translational silencing by alternative 5' transcript leaders.

BACKGROUND: Translational efficiencies in Saccharomyces cerevisiae vary from transcript to transcript by approximately two orders of magnitude. Many of the poorly translated transcripts were found to respond to the appropriate external stimulus by recruiting ribosomes. Unexpectedly, a high frequency of these transcripts showed the appearance of altered 5' leaders that coincide with increased ribosome loading. RESULTS: Of the detectable transcripts in S. cerevisiae, 8% were found to be underloaded with ribosomes. Gene ontology categories of responses to stress or external stimuli were overrepresented in this population of transcripts. Seventeen poorly loaded transcripts involved in responses to pheromone, nitrogen starvation, and osmotic stress were selected for detailed study and were found to respond to the appropriate environmental signal with increased ribosome loading. Twelve of these regulated transcripts exhibited structural changes in their 5' transcript leaders in response to the environmental signal. In many of these the coding region remained intact, whereas regulated shortening of the 5' end truncated the open reading frame in others. Colinearity between the gene and transcript sequences eliminated regulated splicing as a mechanism for these alterations in structure. CONCLUSION: Frequent occurrence of coordinated changes in transcript structure and translation efficiency, in at least three different gene regulatory networks, suggests a widespread phenomenon. It is likely that many of these altered 5' leaders arose from changes in promoter usage. We speculate that production of translationally silenced transcripts may be one mechanism for allowing low-level transcription activity necessary for maintaining an open chromatin structure while not allowing inappropriate protein production.