Tousled-like kinases of maize are up-regulated in dividing tissue and can functionally complement the tousled mutant of Arabidopsis

--Owusu, EO and Rivin, CJ

The Tousled-like kinases (TLKs) are an evolutionarily conserved family of ser/thr kinases that localize to the nucleus. The first member of this gene family was isolated in Arabidopsis thaliana via a T-DNA insertion (Cell 75:939–950). Arabidopsis Tousled (TSL) mutants have a dramatic floral phenotype, showing a decrease in the number of sepals, petals and stamens, and a split gynoecium. Work on TLKs in mammals, C. elegans and Drosophila suggests that TLKs are involved in chromatin remodeling and transcriptional regulation during development. In mammals, activity of TLK proteins is highest during S phase of the cell cycle, but there is not pronounced regulation at the expression level.

In maize, there are 3 TLK genes designated zmTLK1, zmTLK2 and zmTLK3. Like the Arabidopsis gene, the zmTLKs are expressed in all tissues at a low level, and are more highly expressed in floral tissue. To test whether zmTLK expression is generally up-regulated in dividing tissues, expression of the cyclin B and zmTLK genes were examined in multiplex RT-PCR. Both sets of genes are expressed at a higher level in all dividing tissues, i.e. root tip, dividing leaf, young ears and tassels and 9 DAP endosperm as compared to non-dividing tissues like expanding and differentiating leaf sections, silks, culm, and differentiating root sections, though differences in the level of mRNA are more pronounced for cyclinB than for any of the zmTLKs.

All zmTLK genes encode C-terminal catalytic domains with high homology to TSL (84% identity and 93% similarity). The amino acid sequences of the regulatory N-terminal are far less conserved in relation to Arabidopsis, but they share many structural features, such as nuclear localization signals and a series of coiled-coil domains. Using the floral dip method, TSL mutant plants were transformed with and fully complemented by overexpression constructs of zmTLK2, indicating that all the key regions are conserved between the Arabidopsis protein and zmTLK2 and that they function in the same pathway.