A novel form of crosstalk was recently discovered by Zippo and colleagues studying the transcriptional control of FOSL1
, a gene activated in response to serum (Zippo et al., 2009
) (). They present evidence for a transcription activation pathway in which the phosphorylation of H3 tails leads to the acetylation of H4 tails. In turn, acetylation of H4 tails is required for the recruitment of the RNA Pol II positive transcription elongation factor, P-TEFb (). Previously, the authors found that activation of the FOSL1
gene requires PIM1, a proto-oncogene whose kinase activity is activated through MAP kinase signaling. Numerous cellular substrates of PIM1 have been identified, including H3S10. Other H3S10 kinases, such as MSK1 and MSK2 (MSK1/2) are also implicated in the phosphorylation of histones at serum responsive genes, including FOSL1
. Zippo and colleagues find that the spatiotemporal pattern of H3S10 phosphorylation differs for PIM1 and MSK1/2. MSK1/2 mediates the phosphorylation of H3S10 at the promoter of FOSL1
at early time points of gene expression, whereas PIM1 is required for H3S10 phosphorylation at a FOSL1
enhancer after the MSK1/2-mediated phosphorylation of H3S10 ().
Context dependent outcomes of histone crosstalk
Screening for other histone modifications specifically associated with the FOSL1 enhancer shows that the acetylation of H4K16 coincides with H3S10 phosphorylation. RNA interference-mediated knockdown of PIM1 results in loss of H4K16 acetylation, suggesting a trans-tail crosstalk from H3S10 phosphorylation to H4K16 acetylation. Zippo and colleagues asked whether 14-3-3 γ, ε and ζ proteins, previously shown to bind phosphorylated H3S10, are recruited to the promoter and the enhancer of FOSL1 in response to serum. They find that 14-3-3ε and 14-3-3ζ are recruited to both the promoter and enhancer of FOSL1 after serum stimulation. However, 14-3-3 is required only for recruiting the H4K16 histone acetyltransferase MOF to the enhancer, and not to the promoter of FOSL1. Recruitment of MOF to the enhancer results in H4K16 acetylation, which can be bound by the bromodomain-containing protein, Brd4. Brd4 is a component of P-TEFb, a kinase that phosphorylates Pol II to facilitate transcription elongation (). Thus, Zippo and colleagues propose that crosstalk between modifications on two different histone tails regulate productive transcription elongation through the sequential recruitment of proteins that bind these modifications.
One question raised by this study is why H3S10 phosphorylation produces different results at the enhancer than at the promoter of FOSL1
even though 14-3-3 is recruited to both sites. At the enhancer, 14-3-3 recruits the histone acetyltransferase MOF. At the promoter, it does not. What is the difference between 14-3-3 at the promoter and at the enhancer? Interestingly, 14-3-3ε and 14-3-3ζ are thought to be regulated via lysine acetylation (Choudhary et al., 2009
) and an acetyltransferase, Tip60, is preferentially recruited to the promoter of FOSL1
. One possibility is that Tip60 acetylates 14-3-3 and prevents its interaction with MOF.
Another intriguing aspect of the study by Zippo and colleagues is the link between H3S10 phosphorylation and H4K16 acetylation. These two modifications were previously linked in studies of dosage compensation in the fruit fly Drosophila
. In Drosophila
dosage compensation, MOF is recruited to the coding region of X-linked genes in males where it mediates H4K16 acetylation in a process thought to facilitate transcription elongation. Co-localizing with MOF on the male X chromosome is the JIL-1 kinase, an MSK1/2-related kinase that mediates the phosphorylation of H3S10 on this chromosome. In the case of Drosophila
dosage compensation, recruitment of the JIL-1 kinase to the male X chromosome occurs later than H4K16 acetylation (Wang et al., 2001
), a reversal of the order of the addition of these marks at FOSL1
in response to serum. Concordantly, the MOF complex that mediates acetylation in coding regions is likely to be distinct from the MOF complex that mediates promoter/enhancer acetylation of genes (Li et al., 2009
). Thus, by all appearances, these two examples of the coexistence of both H3S10 phosphorylation and H4K16 acetylation are unrelated in their order of implementation and in their biological meaning. This suggests that descriptions of histone modification patterns, without understanding the mechanisms leading to the implementation of these marks, should be interpreted with caution. Importantly, the study by Zippo and colleagues begins to determine the role of histone modifications in the activation of FOSL1
, with a spatial and temporal dissection of how a cascade of histone modifications can lead to a particular transcriptional outcome.