BACKGROUND: The Tousled Like Kinases (TLKs) are involved in chromatin dynamics, 
including DNA replication and repair, transcription, and chromosome segregation. 
Indeed, the first two TLK1 substrates were identified as the histone H3 and Asf1 
(a histone H3/H4 chaperone), which immediately suggested a function in chromatin 
remodeling. However, despite the straightforward assumption that TLK1 acts 
simply by phosphorylating its substrates and hence modifying their activity, 
TLK1 also acts as a chaperone. In fact, a kinase-dead (KD) mutant of TLK1B is 
functional in stimulating chromatin assembly in vitro. However, subtle effects 
of Asf1 phosphorylation are more difficult to probe in chromatin assembly 
assays. Not until very recently was the Asf1 site phosphorylated by TLK1 
identified. This has allowed for probing directly the functionality of a 
site-directed mutant of Asf1 in chromatin assembly assays.
FINDINGS: Addition of either wt or non-phosphorylatable mutant Asf1 to nuclear 
extract stimulates chromatin assembly on a plasmid. Similarly, TLK1B-KD 
stimulates chromatin assembly and it synergizes in reactions with supplemental 
Asf1 (wt or non-phosphorylatable mutant).
CONCLUSIONS: Although the actual function of TLKs as mediators of Asf1 activity 
cannot be easily studied in vivo, particularly since in mammalian cells there 
are two TLK genes and two Asf1 genes, we were able to study specifically the 
stimulation of chromatin assembly in vitro. In such assays, clearly the TLK1 
kinase activity was not critical, as neither a non-phosphorylatable Asf1 nor use 
of the TLK1B-KD impaired the stimulation of nucleosome formation.