A study aimed to understand chromosome segregation based on genetics of Sz. japonicus

In Niki laboratory, we are trying to solve mechanism of chromosome segregation by focusing on unicellular model-organisms, E. coli and fission yeast. We have chosen Schizosaccharomyces japonicus as new fission yeast instead of the conventional budding yeast and fission yeast. There is an advantage in Sz. japonicus that it is easy to observe fine chromosomal dynamics under the microscope, because cellular size of Sz. japonicus is larger than other fission yeasts while genome sizes are comparable. And, nuclear envelope does not generally break during nuclear division in lower eukaryotic cells (closed mitosis), however Sz. japonicus cell has an interesting feature that nuclear envelope partially breaks during nuclear division. To research these features, we have isolated a thousand temperature sensitive mutants of Sz. japonicus, which include mitosis-arrested mutants, defective chromosomal dynamics’ mutants and a less nuclear envelope-breaking mutant. Recently we have also been developing new techniques, vectors that work in Sz. japonicus cell and how to analyze the next generation-sequencing data. We are further going to analyze mutants we found out by using these new techniques in the future.

Regulation for initiation of replication

Chromosome DNA must be precisely replicated for cells to proliferate. In E. coli cells, Assembly of DnaA molecules at origin of replication, oriC, triggers initiation of replication. This initiation process is strictly restricted to occur once and only once at fixed time in cell cycle. Recent studies have shown some mechanisms and factors that regulate the initiation. However, the detailed mechanisms still remain to be elucidated. We are studying the mechanisms which regulate the initiation of DNA replication in E. coli by analyzing the subcellular localization on DnaA protein.

Hyphal photoresponse of Schizosaccharomyces japonicus

Daily changing light affects organisms, not only in plants and animals but also in fungi and bacteria. In fungi, blue light affects development and metabolism through a transcription factor, white collar (WC) complex including flavin. There are two subunits of the WC complex, WC-1 and WC-2 which are found in Neurospora crassa. Orthologs of WC-1 and WC-2 are widely conserved in many filamentous fungi. However, these orthologs are not found in yeast except a few species including Schizosaccharomyces japonicus, which is dimorphic yeast. Dimorphic yeast cells can transit to hyphal cells according to environmental growth conditions. We found that hyphal cells of Sz. japonicus response to blue light. In response to blue light, the cell division cycles of hyphal cells are synchronously activated. There results indicate that the photoresponse of Sz. japonicus is regulated temporally and each hyphal cell synchronously starts and ends cytokinesis. We showed that circadian clock is not involved in this response. However, other type biological clock might be responsible for the photoresponse in order to carry out timely synchronization of cytokinesis in hyphal cells.