Sung-Hoon Jun

DNA mismatch repair system

The molecular mechanisms of the DNA mismatch repair (MMR) system have been uncovered over the last decade, especially in prokaryotes. The results obtained for prokaryotic MMR proteins have provided a framework for the study of the MMR system in eukaryotic organisms, such as yeast, mouse and human, because the functions of MMR proteins have been conserved during evolution from bacteria to humans. However, mutations in eukaryotic MMR genes result in pleiotropic phenotypes in addition to MMR defects, suggesting that eukaryotic MMR proteins have evolved to gain more diverse and specific roles in multicellular organisms. Here, we summarize recent advances in the understanding of both prokaryotic and eukaryotic MMR systems and describe various new functions of MMR proteins that have been intensively researched during the last few years, including DNA damage surveillance and diversification of antibodies.

Molecular cloning and expression analysis of the cell-wall invertase gene family in rice ( Oryza sativa L.)

Cell-wall invertase (CIN) catalyzes the hydrolysis of sucrose into glucose and fructose for the supply of carbohydrates to sink organs via an apoplastic pathway. To study the CIN genes in rice (Oryza sativa L.), we isolated cDNA clones showing amino acid similarity to the plant cell wall invertase proteins from a search of rice sequence databases. Profile analyses revealed that the cloned genes are expressed in unique patterns in various organs. For example, transcripts of OsCIN1, OsCIN2, OsCIN4, and OsCIN7 were detected in immature seeds whereas OsCIN3 gene expression was flower-specific. Further transcript analysis of these genes expressed in developing seeds indicated that OsCIN1, OsCIN2, and OsCIN7 might play an important role involving sucrose partitioning to the embryo and endosperm. Sucrose, a substrate of CINs, induced the accumulation of OsCIN1 transcripts in excised leaves and OsCIN2 in immature seeds, while the level of OsCIN5 was significantly down-regulated in excised leaves treated with sucrose. Infecting the tissues with rice blast (Magnaporthe grisea) as a biotic stressor increased the expression of OsCIN1, OsCIN4, and OsCIN5, suggesting that these genes may participate in a switch in metabolism to resist pathogen invasion. These results demonstrate that OsCIN genes play diverse roles involving the regulation of metabolism, growth, development, and stress responses.

Expressed sequence tags and mRNA expression levels of tagged cDNAs from watermelon anthers and developing seeds

To understand the molecular events that occur during reproductive organ development and to provide genetic resources for molecular breeding, we generated 328 expressed sequence tags (ESTs) from randomly selected clones of four watermelon cDNA libraries. These libraries were prepared from young and mature anthers, as well as the seed coat and inner seed tissues. EST clones found in the young anthers and inner seed tissues showed similarity with genes related to development and signal transduction. We could deduce that, especially in the developing inner seed tissues, important morphological processes were associated exclusively with seed and embryo development In addition, seed metabolism was tailored toward the accumulation of economically valuable storage compounds such as lipids. In the seed coat, EST clones showed similarity with genes that influence the transport or conversion of nutrients such as porin, sucrose synthase, L-asparaginase, and arginine decarboxylase. We also selected two cDNA clones from each of the four classes of ESTs for studying expression levels and patterns in the various organs. Among those eight clones, three (An88, Is124, and Sc68) were expressed preferentially in their particular organ.