Minisymposium 8: Cell division
Abs #
21004: Tousled-like Kinases of maize are up-regulated during endosperm development and can functionally complement the tousled mutant of Arabidopsis
Tousled-like kinases (tlks), are a conserved family of nuclear ser/thr kinases in plants and animals that have been implicated in chromatin remodeling activity during DNA replication and repair: They are maximally active during interphase of the cell cycle; they can phosphorylate histone H3 and interact with Asf1, a histone chaperone that facilitates nucleosome assembly; they are targets of the DNA damage checkpoint kinases ATM and Chk1 (Carrera et al, 2003, Sillje et al, 1999, Groth et al 2003, Sillje and Nigg, 2001). In A. thaliana, mutations in the TOUSLED (TSL) gene cause major abnormalities in floral development, suggesting that it plays a role in modulating gene expression through chromatin remodeling (Roe et al, 1993, 1997).
Zea mays has three tlk genes (ZMTLK1, ZMTLK2, ZMTLK3) encoding similar proteins. These fall into two classes, zmTLK2 and zmTLK3 being more closely related to each other than to zmTLK1. Expression of both classes of zmtlks is ubiquitious, with somewhat higher expression in dividing tissue, including young floral tissue, as for Arabidopsis TSL. In the catalytic region, both zmtlk classes are 84% identical TSL. The N-terminal, regulatory domains are far less conserved in sequence, but have similar structural features including nuclear localization signals and coiled-coil domains required for self-oligomerization. When ZMTLK2 was used to transform Arabidopsis tsl mutant plants, we obtained complete rescue of the tsl mutant phenotype, indicating that the maize protein interacts with appropriate signaling partners for proper enzymatic and developmental function.
ZMTLK2/3 gene expression is dramatically increased during endosperm development. This increase occurs during the period of genome endoreduplication in the endosperm, a process that results in very high ploidy levels and alterations in chromatin structure (Zhao and Grafi, 2002). Both classes of ZMTLK s may be part of a general system for chromatin remodeling during replication, but ZMTLK2/3 may have a specific role in endoreduplication. Some possible functions for endosperm ZMTLK2/3: The increased gene expression could reflect a requirement a high rate for new chromatin assembly during reiterative DNA synthesis, or alterations in chromatin structure might be required to permit re-initiation of synthesis at replication origins. A third possibility is that altered chromatin structure is needed for the high biosynthetic activity of cells in this storage tissue.
