A Family of Transcriptional Co-repressors Controlling Multiple Processes in Plants

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Living organisms need to turn expression of their genes on and off in response to their environment. Transcription factors bind to specific DNA sequences and control whether genes are activated or repressed. Turning off gene expression is not simply the absence of activation: actively shutting down expression of target genes is of vital importance. For example, in this proposal we will look at two cases where repressing target genes is required for essential plant processes. In one example, we will investigate the role of repression in hormone signalling and in the other we will look at the role of repression in maintaining the pool of undifferentiated cells at the growing tip of the shoot (the meristem) that is essential for plants to modulate their development in response to age and environment. Transcription factors repress gene expression using a class of proteins called transcriptional co-repressors. These proteins cannot bind DNA and must interact with transcription factors to be recruited to the regulatory regions of genes. Target gene specificity is contributed by the transcription factor and the repressive capacity is provided by the co-repressor. We identified the first members of a small family of plant co-repressors that here we call the TTW family. There are five TTW proteins in the model plant Arabidopsis. Others have shown that this family is required as a central part of the signalling mechanism for the plant hormone auxin. The ability of each cell to perceive its auxin environment and to regulate expression of its genes in response to that environment plays a huge part in shaping the plant and allowing it to function. Understanding how auxin levels are translated into instructions to turn genes on and off has been one of the major scientific achievements of the last 5 years. We now know that one of the key players in this signalling process is able to shut down gene expression by recruiting members of the TTW family. The TTW family is therefore already linked directly to two distinct processes that are central to plant life: meristem maintenance and auxin signalling. We have new data that indicate that the influence of the TTW family is much wider than this. Our evidence shows that just one member of the TTW family helps a minimum of 110 different transcription factors to repress target genes. It also implicates the TTW family in another phytohormone signalling pathway; suggesting that the remarkable organisational similarities between the jasmonic acid and auxin signalling pathways extend to a common use of this class of co-repressors. Given that many aspects of gene regulation appear to converge on this small family of proteins it is important that we understand how they work. To this end we propose three objectives that address the following questions: 1. Is the TTW family of co-repressors required to enact the repressive function in the jasmonic acid signalling pathway, as it is in the auxin signalling pathway? 2. Can we use the connection between the TTW corepressors and meristem maintenance to understand the mode of action of the TTW co-repressors? 3. Can we define the full extent of the interactions between the TTW proteins and plant transcription factors so that we can define exactly what allows any transcription factor to recruit a TTW co-repressor? The first question allows us to understand more about a vital hormone signalling pathway and reveals a remarkable evolutionary similarity between the way in which different proteins become organised in a similar way. The second question, explains the link between meristem maintenance and the TTW proteins, and shows how the TTW proteins actively shut down gene expression. The final question will produce a community resource to catalogue TTW - transcription factor interactions and should also help us to define the rules that determine which transcription factors interact with which co-repressors.