Cell Cycle Genes

The sequencing of the O. tauri genome has allowed the analysis of core cell cycle genes. This analysis revealed that, like C. reinhardtii, the O. tauri genome encodes one homolog of each group of CDKs and cyclins including a plant-specific CDK of B-type (Robbens et al. 2005). The expression analysis of some cell cycle genes showed that CDKA, CYCA, and CYCD are transcribed ubiquitously during the cell cycle while mRNA for CDKB and CYCB accumulate only during S and M phases (Corellou et al. 2005; Farinas et al. 2006). The expression pattern therefore mimics that of the corresponding homologs in Arabidopsis thaliana and C. reinhardtii (Segers et al. 1996; Mironov et al. 1999; Bisova et al. 2005). O. tauri CDKB has been characterized in more detail (Corellou et al. 2005). This study has shown that CDKB protein is present only during S and M phases and CDKB (and not CDKA) is also regulated by tyrosine phosphorylation. In line with the nature of tyrosine phosphorylation in other systems, this modification has an inhibitory effect and is removed prior to mitosis leading to the activation of CDKB/cyclin complex. CDKB his-

tone H1 kinase activity is about tenfold higher than that of CDKA; also CDKB is responsible for the main peak of mitotic activity, while CDKA has a more steady-state activity that probably accounts for the regulation of S phase. Therefore it seems that in O. tauri, CDKB is the main mitotic kinase while in higher plants this type of kinase has evolved to fill in more specialized functions outside the cell cycle (Boudolf et al. 2004). It would be interesting to see what is the function of CDKB in C. reinhardtii to discriminate which of these two scenarios represents the ancestral state.

O. tauri is interesting in another aspect of the cell cycle regulation because it is the first member of the green lineage to encode a functional Cdc25 phosphatase (Khadaroo et al. 2004). While CDKs, cyclins, and Wee1 kinases are present in all eukaryotes, there is no distinguishable homolog of Cdc25 phosphatase in higher plants, a fact that has been puzzling researchers in the field for years. The O. tauri genome encodes a protein whose sequence shows clear similarity to Cdc25 phosphatases from yeast and animals. Moreover, this protein complements a cdc25 mutation in Schizosaccharomyces pombe and activates CDK1/cyclin B complexed from starfish oocytes both in vitro and in vivo (Khadaroo et al. 2004). The sequence of O. tauri Cdc25 phos-phatase shares Cdc25 features in its C terminus but diverges from yeast and animal homologs in its N terminus. Using O. taurfs Cdc25 a putative Cdc25-like protein can be found in C. reinhardtii (Khadaroo et al. 2004; Bisova et al. 2005) and also in Arabidopsis (Landrieu et al. 2004a,b); however, it will require a functional verification to prove that any of these putative candidates have Cdc25 phosphatase activity.

As mentioned above, O. tauri is a promising new model for cell cycle studies. The main advantage of this system is its evolutionary position at the base of green lineage. O. tauri has already proven its usefulness as an intermediate model between plants and fungi/animals. Although no stable DNA transformation has been reported, one can assume that, as in the case of S. quadricauda, techniques used for C. reinhardtii or C. merolae (see below) may be successful.

0 0

Post a comment