Graduate Thesis Or Dissertation
 

Characterization of the Zea mays ssp. mays TOUSLED-like kinases

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  • This dissertation describes the cloning and characterization of the TOUSLEDlike kinases genes of maize (ZmTLKs). The TOUSLED-like kinases (TLKs) are a conserved family of nuclear Ser/Thr kinases in higher eukaryotes. The maize genome has three TOUSLED-like kinase genes (ZmTLK1, ZmTLK2, and ZmTLK3). Based upon sequence similarity, the ZmTLKs are divided into two classes, the ZmTLK1 and the ZmTLK2/3 class. The origins of these genes can be inferred from their map positions and relationships with TLKs in other Zea species. The ZmTLK1 and ZmTLK2 genes occupy syntenous positions on chromosome arms 1L and 5S in the maize genome. There are two equivalent classes of TLK genes in other Zea species, altogether indicating that the two ZmTLK classes are orthologous genes from the precursor species of maize, an ancient allotetraploid. Gene expression studies of ZmTLKs show that there is a higher level of expression in tissues undergoing DNA synthesis. This is consistent with studies of TLKs in animal systems that show involvement in chromatin assembly/remodeling activities during DNA replication and repair, as well as in transcription. The highest level of gene expression for the ZmTLK2/3 class was observed during development of the endosperm, in a period of massive nuclear endoreduplication. ZmTLK1 is not upregulated in endoreduplicating endosperm, suggesting functional divergence between the two classes of ZmTLK genes. The function of the ZmTLKs was examined by testing whether maize TLK genes could complement the tousled mutant of Arabidopsis. In Arabidopsis thaliana, recessive mutations in the single copy TOUSLED (TSL) gene cause moderate vegetative and severe floral defects, suggesting that TLKs may play a role in gene expression modulation through chromatin remodeling. The ZmTLK proteins are 84% identical to TSL in the catalytic region and 45 - 49% at the N-terminal regulatory domain. However, structural features of the N-terminal region domains of the ZmTLKs are similar to that of TSL. Arabidopsis tsl-1 mutant plants were transformed with ZmTLK2, under the control of the CaMV 35S promoter. These plants showed wild-type Arabidopsis phenotype, indicating that in spite of their sequence differences, ZmTLK2 and TSL interact with the same substrates and regulatory partners and are functionally equivalent.
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