TLK1B promotes repair of UV-damaged DNA through chromatin remodeling by Asf1

We now show that TLK1B also affords protection against UV radiation. We find that nuclear extracts isolated from TLK1B-containing mouse cells promote more efficient chromatin assembly than comparable extracts lacking TLK1B. TLK1B-containing extracts are also more efficient in repair of UV-damaged plasmid DNA assembled into nucleosomes. One of the two known substrates of TLK1 (or TLK1B) is the histone chaperone Asf1, and immuno-inactivation experiments suggest that TLK1B increases UV-repair through the action of Asf1 on chromatin assembly/disassembly.Our studies provide evidence for TLK1B-mediated phosphorylation of Asf1 triggering DNA repair. We suggest that this occurs via Asf1-mediated chromatin assembly at the sites of UV damage.Tousled-like kinases (TLKs) belong to a family of serine-threonine kinases highly conserved in plants as well as animals [1,2]. Tousled-mutants in Arabidopsis have abnormal flower development, with defects also in leaf morphology and flowering time [3,4]. Humans have two homologs of Tousled: TLK1 and TLK2 [2]. The exact function of these kinases has not been determined, but they are known to act in a cell-cycle dependent manner. They are maximally active during the S phase, and are also the targets of checkpoint kinases [5]. Specifically, it has been reported that TLK1 is inhibited by ATM-Chk1 by direct phosphorylation at Ser 695 [6]. Knockout of the Tousled gene in Drosophila and C. elegans cause an early arrest in embryonic development [7,8], while expression of a dominant negative mutant in mouse cells causes loss of nuclear divisions and missegregation of chromosomes [9]. The importance of TLK1 in chromosome segregation was also confirmed by a study on C. elegans embryos [10]. We recently cloned a cDNA encoding a mammalian Tousled-like kinase, through a different scheme, based on the polysomal redistribution of weakly translated transcripts that become preferentially recruited upon overexpression of the translation initiation factor e