Glenn Croston and his colleagues studied the relation between chromatin structure and transcription activity. In one set of experiments, they measured the level of in vitro transcription of a Drosophila gene by RNA polymerase II in the presence of DNA and various combinations of histone proteins (G. E. Croston et al. 1991. Science 251:643–649). First, they measured the level of transcription of naked DNA, with no associated histone proteins. Then they measured the level of transcription after nucleosome octamers (without H1) were added to the DNA. The addition of the octamers caused the level of transcription to drop by 50%. When both nucleosome octamers and H1 proteins were added to the DNA, transcription was greatly repressed, dropping to less than 1% of that obtained with naked DNA, as shown in the table below. GAL4-VP16 is a protein that binds to the DNA of certain eukaryotic genes. When GAL4-VP16 is added to DNA, the level of transcription by RNA polymerase II is greatly elevated. Treatment Relative amount of transcription Naked DNA 100 DNA + octamers 50 DNA + octamers + H1 <1 DNA + GAL4-VP16 1000 DNA + octamers + GAL4VP16 1000 DNA + octamers + H1 + GAL4-VP16 1000 Even in the presence of the H1 protein, GAL4-VP16 stimulates high levels of transcription. Propose a mechanism by which the H1 protein represses transcription and by which GAL4-VP16 overcomes this repression. Explain how your proposed mechanism would produce the results obtained in these experiments.
Glenn Croston and his colleagues studied the relation between chromatin structure and transcription activity. In one set of experiments, they measured the level of in vitro transcription of a Drosophila gene by RNA polymerase II in the presence of DNA and various combinations of histone proteins (G. E. Croston et al. 1991. Science 251:643–649). First, they measured the level of transcription of naked DNA, with no associated histone proteins. Then they measured the level of transcription after nucleosome octamers (without H1) were added to the DNA. The addition of the octamers caused the level of transcription to drop by 50%. When both nucleosome octamers and H1 proteins were added to the DNA, transcription was greatly repressed, dropping to less than 1% of that obtained with naked DNA, as shown in the table below. GAL4-VP16 is a protein that binds to the DNA of certain eukaryotic genes. When GAL4-VP16 is added to DNA, the level of transcription by RNA polymerase II is greatly elevated.
Treatment Relative amount of transcription
Naked DNA 100
DNA + octamers 50
DNA + octamers + H1 <1
DNA + GAL4-VP16 1000
DNA + octamers + GAL4VP16 1000
DNA + octamers + H1 + GAL4-VP16 1000
Even in the presence of the H1 protein, GAL4-VP16 stimulates high levels of transcription. Propose a mechanism by which the H1 protein represses transcription and by which GAL4-VP16 overcomes this repression. Explain how your proposed mechanism would produce the results obtained in these experiments.
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