BLM, the Bloom's syndrome protein, varies during the cell cycle in its amount, distribution, and co-localization with other nuclear proteins

BLM, the protein encoded by the gene mutated in Bloom's syndrome (BS), is a phylogenetically highly conserved DNA helicase that varies in amount and d istribution in the nucleus during the cell-division cycle. It is undetectab le in many cells as they emerge from mitosis but becomes abundant during G( 1) and remains so throughout S, G(2), and mitosis. BLM is widely distribute d throughout the nucleus but at certain times also becomes concentrated in foci that vary in number and size. It co-localizes transitorily with replic ation protein A (RPA) and promyelocytic leukemia protein (PML) nuclear bodi es, and at times it enters the nucleolus. The observations support the hypo thesis that BLM is distributed variously about the nucleus to manipulate DN A in some, very possibly several, nucleic acid transactions, when and where they take place. The specific transaction(s) remain to be identified. Alth ough absence from the nucleus of functional BLM - the situation in BS - obv iously is not lethal in the human, other helicases would appear to be unabl e to substitute for it completely, witness the hypermutability and hyperrec ombinability of BS cells. Copyright (C) 2001 S. Karger AG, Basel.