The ternary complex factor (TCF) subfamily of ETS-domain transcription factors bind with serum response factor (SRF) to the serum response element (SRE) and mediate increased gene expression. The TCF protein Elk-1 is phosphorylated by the JNK and ERK groups of mitogen-activated protein (MAP) kinases causing increased DNA binding, ternary complex formation, and transcriptional activation. Activated SRE-dependent gene expression is induced by JNK in cells treated with interleukin-1 and by ERK after treatment with phorbol ester. The Elk-1 transcription factor therefore integrates MAP kinase signaling pathways in vivo to coordinate biological responses to different extracellular stimuli.

The p16INK4a cyclin-dependent kinase inhibitor is implicated in replicative senescence, the state of permanent growth arrest provoked by cumulative cell divisions or as a response to constitutive Ras-Raf-MEK signalling in somatic cells. Some contribution to senescence presumably underlies the importance of p16INK4a as a tumour suppressor but the mechanisms regulating its expression in these different contexts remain unknown. Here we demonstrate a role for the Ets1 and Ets2 transcription factors based on their ability to activate the p16INK4a promoter through an ETS-binding site and their patterns of expression during the lifespan of human diploid fibroblasts. The induction of p16INK4a by Ets2, which is abundant in young human diploid fibroblasts, is potentiated by signalling through the Ras-Raf-MEK kinase cascade and inhibited by a direct interaction with the helix-loop-helix protein Id1 (ref. 11). In senescent cells, where the Ets2 levels and MEK signalling decline, the marked increase in p16INK4a expression is consistent with the reciprocal reduction of Id1 and accumulation of Ets1.

The MADS-box family of transcription factors has been defined on the basis of primary sequence similarity amongst numerous proteins from a diverse range of eukaryotic organisms including yeasts, plants, insects, amphibians and mammals. The MADS-box is a conserved motif found within the DNA-binding domains of these proteins and the name refers to four of the originally identified members: MCM1, AG, DEFA and SRF. Several proteins within this family have significant biological roles. For example, the human serum-response factor (SRF) is involved in co-ordinating transcription of the protooncogene c-fos, whilst MCM1 is central to the transcriptional control of cell-type specific genes and the pheromone response in the yeast Saccharomyces cerevisiae. The RSRF/MEF2 proteins comprise a sub-family of this class of transcription factors which are key components in muscle-specific gene regulation. Moreover, in plants, MADS-box proteins such as AG, DEFA and GLO play fundamental roles during flower development. The MADS-box is a contiguous conserved sequence of 56 amino acids, of which 9 are identical in all family members described so far. Several members have been shown to form dimers and consequently two functional regions within the MADS-box have been defined. The N-terminal half is the major determinant of DNA-binding specificity whilst the C-terminal half is necessary for dimerisation. This organisation allows the potential formation of numerous proteins, with subtly different DNA-binding specificities, from a limited number of genes by heterodimerisation between different MADS-box proteins. The majority of MADS-box proteins bind similar sites based on the consensus sequence CC(A/T)6GG although each protein apparently possesses a distinct binding specificity. Moreover, several MADS-box proteins specifically recruit other transcription factors into multi-component regulatory complexes. Such interactions with other proteins appears to be a common theme within this family and play a pivotal role in the regulation of target genes.

Recently, SUMO modification has been shown to impart repressive properties on several transcriptional regulatory proteins. Indeed, the ETS domain transcription factor Elk-1 is modified by SUMO, and this modification is reversed by ERK MAP kinase pathway activation. This causes a switch from a repressive to activated state. However, the mechanism(s) of SUMO-mediated transcriptional repression is unclear. Here, we have investigated how sumoylation of Elk-1 leads to transcriptional repression. We demonstrate that sumoylation of Elk-1 results in the recruitment of histone deacetylase activity to promoters. In particular, our data point to a key role for HDAC-2. This recruitment leads to decreased histone acetylation and hence transcriptional repression at Elk-1 target genes. Thus, our data demonstrate an important integration point for two protein-modifying pathways in the cell, the SUMO and deacetylation pathways, that combine to promote transcriptional repression.

The MADS‐box family of transcription factors has been defined on the basis of primary sequence similarity amongst numerous proteins from a diverse range of eukaryotic organisms including yeasts, plants, insects, amphibians and mammals. The MADS‐box is a conserved motif found within the DNA‐binding domains of these proteins and the name refers to four of the originally identified members: MCM1, AG, DEFA and SRF. Several proteins within this family have significant biological roles. For example, the human serum‐response factor (SRF) is involved in co‐ordinating transcription of the proto‐oncogene c‐fos, whilst MCM1 is central to the transcriptional control of cell‐type specific genes and the pheromone response in the yeast Saccharomyces cerevisiae. The RSRF/MEF2 proteins comprise a subfamily of this class of transcription factors which are key components in muscle‐specific gene regulation. Moreover, in plants, MADS‐box proteins such as AG, DEFA and GLO play fundamental roles during flower development. The MADS‐box is a contiguous conserved sequence of 56 amino acids, of which 9 are identical in all family members described so far. Several members have been shown to form dimers and consequently two functional regions within the MADS‐box have been defined. The N‐terminal half is the major determinant of DNA‐binding specificity whilst the C‐terminal half is necessary for dimerisation. This organisation allows the potential formation of numerous proteins, with subtly different DNA‐binding specificities, from a limited number of genes by heterodimerisation between different MADS‐box proteins. The majority of MADS‐box proteins bind similar sites based on the consensus sequence CC(A/T)6 GG although each protein apparently possesses a distinct binding specificity. Moreover, several MADS‐box proteins specifically recruit other transcription factors into multi‐component regulatory complexes. Such interactions with other proteins appears to be a common theme within this family and play a pivotal role in the regulation of target genes.