11. Polycomb and trithorax are antagonistic repressors and activators

21.11 Polycomb and trithorax are antagonistic repressors and activators


Chromatin can be specifically repressed. One example is the formation of heterochromatin (see 18 Chromosomes). Another is provided by the genetics of homeotic genes in Drosophila, which have led to the identification of a protein complex that may maintain certain genes in a repressed state. Pc mutants show transformations of cell type that are equivalent to gain-of-function mutations in Antennapedia (Antp) or Ultrabithorax, because these genes are expressed in tissues in which usually they are repressed. This implicates Pc in regulating transcription. Furthermore, Pc is the prototype for a class of loci called the Pc group (Pc-G); mutations in these genes generally have the same result of derepressing homeotic genes, suggesting the possibility that the group of proteins has some common regulatory role. A connection between chromatin remodeling and repression is indicated by the properties of brahma, a fly counterpart to SWI2. Loss of brahma function suppresses mutations in Polycomb.


Consistent with the pleiotropy of Pc mutations, Pc is a nuclear protein that can be visualized at ~80 sites on polytene chromosomes. These sites include the Antp gene. Another member of the Pc-G, polyhomeotic, is visualized at a set of polytene chromosome bands that are identical with those bound by Pc. The two proteins coimmunoprecipitate in a complex of ~2.5 106 D that contains 10 V15 polypeptides. The relationship between these proteins and the products of the ~30 Pc-G genes remains to be established; one possibility is that many of these gene products form a general repressive complex that is modified by some of the others for specific loci (Franke et al., 1992; Zink and Paro, 1989).




Figure 21.21 Pc-G proteins do not initiate repression, but are responsible for maintaining it.

The Pc-G proteins are not conventional repressors. They are not responsible for determining the initial pattern of expression of the genes on which they act. In the absence of Pc-G proteins, these genes are initially repressed as usual, but later in development the repression is lost without Pc-G group functions. This suggests that the Pc-G proteins in some way recognize the state of repression when it is established, and they then act to perpetuate it through cell division of the daughter cells. Figure 21.21 shows a model in which Pc-G proteins bind in conjunction with a repressor, but the Pc-G proteins remain bound after the repressor is no longer available. This is necessary to maintain repression, so that if Pc-G proteins are absent, the gene becomes activated (Eissenberg et al., 1990).


A region of DNA that is sufficient to enable the response to the Pc-G genes is called a PRE (Polycomb response element). It can be defined operationally by the property that it confers repression of enhancers in its vicinity throughout development. The assay for a PRE is to insert it close to a reporter gene that is controlled by an enhancer that is repressed in early development, and then to determine whether the reporter becomes expressed subsequently in the descendants. An effective PRE will prevent such re-expression (Chan et al., 1994).


The PRE is a complex structure, ~10 kb. No individual member of the Pc-G proteins has yet been shown to bind to specific sequences in the PRE, so the basis for the assembly of the complex is still unknown. When a locus is repressed by Pc-G proteins, however, the proteins appear to be present over a much larger length of DNA than the PRE itself. Polycomb is found locally over a few kilobases of DNA surrounding a PRE.




Figure 19.45 Extension of heterochromatin inactivates genes. The probability that a gene will be inactivated depends on its distance from the heterochromatin region.

This suggests that the PRE may provide a nucleation center, from which a structural state depending on Pc-G proteins may propagate. This model is supported by the observation of effects related to position effect variegation (see Figure 19.45), that is, a gene near to a locus whose repression is maintained by Pc-G may become heritably inactivated in some cells but not others. In one typical situation, crosslinking experiments in vivo showed that Pc protein is found over large regions of the bithorax complex that are inactive, but the protein is excluded from regions that contain active genes. The idea that this could be due to cooperative interactions within a multimeric complex is supported by the existence of mutations in Pc that change its nuclear distribution and abolish the ability of other Pc-G members to localize in the nucleus. The role of Pc-G proteins in maintaining, as opposed to establishing, repression must mean that the formation of the complex at the PRE also depends on the local state of gene expression (Orlando and Paro, 1993).


A connection between the Pc-G complex and more general structural changes in chromatin is suggested by a homology between a 37 amino acid region near the N-terminus of Pc and a nonhistone protein, HP1, that is associated with heterochromatin. The common motif is called the chromodomain. HP1 is coded by the gene Su(var)205, a suppressor of position-effect variegation. Since variegation is probably caused by the spreading of inactivity from constitutive heterochromatin, it is possible that the chromodomain is used by Pc and HP1 to interact with common components that are involved in inducing the formation of heterochromatic or inactive structures. This model implies that similar mechanisms are used to repress individual loci or to create heterochromatin.


The trithorax group (trxG) of proteins have the opposite effect to the Pc-G proteins: they act to maintain genes in an active state. There may be some similarities in the actions of the two groups: mutations in some loci prevent both Pc-G and trx from functioning, suggesting that they could rely on common components. A factor coded by the trithorax-like gene, called GAGA because it binds to GA-rich consensus sequences, has binding sites in the PRE. In fact, the sites where Pc binds to DNA coincide with the sites where GAGA factor binds (Geyer and Corces, 1992; Strutt et al., 1997).


What does this mean? GAGA is probably needed for activating factors, including trxG members, to bind to DNA. The genetics suggest that it is needed for activation by trxG. Is it also needed for PcG proteins to bind and exercise repression? This is not yet clear, but such a model would demand that something other than GAGA determines whether PcG or trxG complexes subsequently assemble at the site.



Research
Chan, C.-S., Rastelli, L., and Pirrotta, V. (1994). A Polycomb response element in the Ubx gene that determines an epigenetically inherited state of repression. EMBO J. 13, 2553-2564.
Eissenberg, J. C., James, T. C., Fister-Hartnett, D. M., Hartnett, T., Ngan, V., and Elgin, S. C. R. (1990). Mutation in a heterochromatin-specific chromosomal protein is associated with suppression of position-effect variegation inD. melanogaster. Proc. Nat. Acad. Sci. USA 87, 9923-9927.
Franke, A., DeCamillis, M., Zink, D., Cheng, N., Brock, H. W., and Paro, R. (1992). Polycomb and polyhomeotic are constituents of a multimeric protein complex in chromatin ofD. melanogaster. EMBO J. 11, 2941-29.
Geyer, P. K. and Corces, V. G. (1992). DNA position-specific repression of transcription by a Drosophila zinc finger protein. Genes Dev. 6, 1865-1873.
Orlando, V. and Paro, R. (1993). Mapping Polycomb-repressed domains in the bithorax complex using in vivo formaldehyde cross-linked chromatin. Cell 75, 1187-1198.
Strutt, H., Cavalli, G., and Paro, R. (1997). Colocalization of Polycomb protein and GAGA factor on regulatory elements responsible for the maintenance of homeotic gene expression. EMBO J. 16, 3621-3632.
Zink, B. and Paro, R. (1989). In vivo binding patterns of a trans-regulator of the homeotic genes inD. melanogaster. Nature 337, 468-471.



Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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