Abstract
Chromodomains in a variety of proteins mediate the formation of heterochromatin by interacting directly with histone H3, DNA, or RNA. A diverse family of long terminal repeat (LTR)-retrotransposons possesses chromodomains in their integrases (IN), suggesting that the chromodomains may control integration. The LTR-retrotransposon Tf1 of Schizosaccharomyces pombe is highly active and possesses a chromodomain in the COOH terminus of its IN. To test this chromodomain for a role in integration, recombinant INs with and without the chromodomain were assayed for activity in in vitro reactions. The full-length IN had integration activity with oligonucleotide substrates that modeled both the insertion reaction and a reverse reaction known as disintegration. The INs of retroviruses possess an additional activity termed 3' processing that must remove 2-3 nucleotides from the 3' ends of the viral cDNA before insertion can occur. These additional nucleotides are added during reverse transcription because of the position of the minus strand primer downstream of the LTR. The position of the primer for Tf1 suggests no nucleotides are added 3' of the LTR. It was therefore surprising that Tf1 IN was capable of 3' cleavage. The most unexpected result reported here was that the IN lacking the chromodomain had significantly higher activity and substantially reduced substrate specificity. These results reveal that both the activity and specificity of enzymes can be modulated by their chromodomains.
Highlights
Post-translational modifications of histones, such as the acetylation or methylation of lysines, establish the accessibility of DNA to the transcription machinery [1,2,3]
The CA dinucleotides are conserved among all retroviruses and their position at the 3Ј ends of the cDNA is required for the second step of retrovirus integration, the “strand transfer reaction.”
The central domain of HIV-1 IN was shown to be sufficient for catalytic activity in in vitro assays for the reverse reaction of strand transfer known as disintegration [17]
Summary
Post-translational modifications of histones, such as the acetylation or methylation of lysines, establish the accessibility of DNA to the transcription machinery [1,2,3]. The minus strand primers initiate reverse transcription 2 or 3 nucleotides from the LTR and this results in additional nucleotides at the cDNA termini that IN removes during the processing reaction.
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