Cells respond to changes in their environment through activation of genetic regulatory networks, groups of genes with mutual regulatory interactions. Coactivated or repressed genes and their specific regulators function together, altering the cellular composition to cope with a varying environment (18
). Upon reception of intrinsic or extrinsic cues, cellular needs are met both through production of new proteins and through silencing the expression of genes whose respective protein products are not needed or would be deleterious to the cell during a particular developmental program or environmental condition. Down-regulation of transcript levels can be mediated through multiple mechanisms of transcriptional repression (31
) and though regulated degradation (37
). Furthermore, translation of transcripts can be silenced, either directly (29
) or through alterations of transcript structures (29
) resulting in either exposure of elements inhibitory to translation (26
) or exclusion of translational start sites (29
Our previous work revealed the frequent occurrence of altered 5′ termini among transcripts during cellular responses to new environmental conditions (29
). In every case, one of the transcript forms was inefficiently translated, while the other was efficiently loaded with ribosomes. For some, the undertranslated transcript possessed a long 5′ leader originating from an upstream intergenic start site suggesting the presence of an independent promoter. However, several of the transcripts were shortened under the new conditions with their 5′ terminus residing within the open reading frame (ORF), in turn preventing the production of the full-length protein.
In order to better understand the processes by which these 5′ structural alterations occur, we analyzed the mechanism(s) responsible for producing, in response to Saccharomyces cerevisiae
mating pheromone, an altered PRY3
transcript with its 5′ terminus residing within the ORF at +452 relative to the translational initiation codon of the full-length protein. Pry3p is a cell wall protein (56
) of unknown function but whose transcript is cell cycle regulated with the “SIC1
). The promoter region of PRY3
contains putative binding sites for either the daughter cell-specific transcription factor Ace2p or the non-cell-specific activator Swi5p (11
). The PRY3
promoter region directs reporter expression only in daughter cells (9
), suggestive of a role specifically in their cell wall structure.
Here we report that the pheromone-responsive transcription factor Ste12p and its binding sites near the TATA box upstream of the PRY3 gene impede transcription from the full-length start site and in the process activate an intragenic promoter, which generates the short PRY3 transcript. The daughter-specific transcription factor Ace2p is required for active transcription from both full-length and internal start sites, and since full-length PRY3 expression is inhibitory to mating, repression by Ste12p must optimize the mating competence of newborn daughter cells. The multiple elements within the PRY3 locus are a clear example of local competition providing a platform for coordinating inductive and repressive activities within a regulatory network, in this instance the response to pheromone.