DNA replication: Sir2 finds its voice in replication
Researchers have identified a new role for the histone deacetylase Sir2 that is independent of its well—known roles in silencing–the negative regulation of the initiation of DNA replication in Saccharomyces cerevisiae.
Reporting in Genes & Development, Michael Weinreich and colleagues have identified a new role for the histone deacetylase Sir2 that is independent of its well-known roles in silencing — the negative regulation of the initiation of DNA replication in Saccharomyces cerevisiae.
The authors wanted to understand more about the assembly of pre-replicative complexes (pre-RCs), which establish replication competence. In S. cerevisiae, pre-RCs consist of Cdc6, the origin recognition complex, Cdt1 and the MCM helicase: once assembled they ultimately promote the formation of bidirectional replication forks.
They found that the deletion of silent information regulator SIR2 suppressed a cdc6-4 replication-initiation mutant. But this was not indirect suppression through the loss of silencing or due to the loss of recombination. Using cdc6-4 sirN345A double mutants — in which a single amino acid in the catalytic domain of Sir2 is mutated to abolish its deacetylase activity — Weinreich and colleagues found that it was the loss of enzymatic activity that suppressed the cdc6-4 mutant. And this effect was not due to the stabilization of Cdc6 or because the requirement for CDC6 was bypassed when SIR2 was deleted.
Deleting SIR2 rescued the initiation defect of cdc6-4; it also rescued other initiation mutants, but only those affecting pre-RC assembly. In addition, in cdc6-4 mutants, the deletion of SIR2 rescued the high rate of plasmid loss that is typical of initiation mutants. Finally, deleting SIR2 was found to promote the binding of MCM proteins to origins of replication in the cdc6-4 mutant.
But what does this mean? The authors suggest that, in S. cerevisiae, Sir2 inhibits the initiation of DNA replication at a step early in pre-RC assembly through its activity as a protein deacetylase: it will be interesting to see if Sir2 inhibits initiation through its known activity as a histone deacetylase or perhaps by deacetylating non-histone proteins as mammalian SIR2 orthologues have been shown to do.
So, this work has probably not only uncovered a new mechanism for the negative regulation of the initiation of DNA replication, it also provides another connection between the proteins that are involved in forming silent heterochromatin and initiating DNA replication. Weinreich and colleagues conclude by speculating that if SIR2 functions in metazoans in the same way as it does in S. cerevisiae, then it could be part of a mechanism that links "... replication domains with heritable transcriptional states during development or in response to cell autonomous signals."
Natalie Wilson
References
- Pappas, D. L. Jr, Frisch, R. & Weinreich, M. The NAD+-dependent Sir2p histone deacetylase is a negative regulator of chromosomal DNA replication. Genes Dev. 18, 769–781 (2004) | Article | PubMed |
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