Recently published structural and functional analyses of the CMG complex have provided insight into the mechanism of its DNA helicase function and into the distinct roles of its central six component proteins MCM2-MCM7 (MCM2-7). To activate CMG helicase, the two protein kinases CDK and DDK, as well as MCM10, are required. In addition to the initiation of DNA replication, MCM function must be regulated at the DNA replication steps of elongation and termination. Polyubiquitylation of MCM7 is involved in terminating MCM function. Reinitiation of DNA replication in a single cell cycle, which is prevented mainly by CDK, is understood at the molecular level. MCM2-7 gene expression is regulated during cellular aging and the cell cycle, and the expression depends on oxygen concentration. These regulatory processes have been described recently. Genomic structural alteration, which is an essential element in cancer progression, is mainly generated by disruptions of DNA replication fork structures. A point mutation in MCM4 that disturbs MCM2-7 function results in genomic instability, leading to the generation of cancer cells. In this review, I focus on the following points: 1) function of the MCM2-7 complex, 2) activation of MCM2-7 helicase, 3) regulation of MCM2-7 function, 4) MCM2-7 expression, and 5) the role of MCM mutation in cancer progression.

Dihydrosphingosine C4 hydroxylase (DSH), a diiron-binding membrane enzyme, catalyzes the hydration of dihydrosphingosine and acyl-sphinganine to produce phytosphingosine and phytoceramide, respectively. Rice has two types of DSH homologs: general DSHs, namely DSH1, DSH2 and DSH4, and others that show spatial expression profiles, namely DSH3 and DSH5. The general DSHs exist in many plant species. These DSHs showed similarity in their functions and complemented the yeast sur2D mutation. In contrast, homologs of DSH3 and DSH5 were found only in monocot plants. Phylogenetic analysis placed these DSHs in different clades that are evolutionarily divergent from those of the general DSHs. DSH3 and DSH5 showed low-level expression. DSH5 expression was specifically in vascular bundle tissues. Ectopic expression of DSH5 induced a dwarf phenotype characterized by severe growth inhibition and an increase in the thickness of the leaf body caused by enlargement of bulliform cells in the leaves. However, no significant difference was observed in the amount of sphingolipid species. DSH5 did not complement the yeast sur2D mutation, implying that DSH5 has little effect on sphingolipid metabolism. These findings suggested that DSH3 and DSH5 originated and diverged in monocot plants.

Cytoplasmic male sterility (CMS) is an agronomically important trait whose causative genes are located in the mitochondrial genome. A CMS rapeseed, Brassica napus ‘SW18’, was made 25 years ago by an asymmetric (or “donor-recipient”) cell fusion between B. napus ‘Westar’ and a CMS radish (Raphanus sativus ‘Kosena’), in order to transfer the radish CMS-associated gene without disturbing the rapeseed features. Here, we determined the nucleotide sequences of the mitochondrial genomes of Kosena radish and SW18. SW18 has a recombinant mitochondrial genome, which includes the whole 222-kb genome of Westar (54 genes) and a total of 23 kb insertions of four fragments from Kosena radish (three genes: orf125, trnfM and atp1). All of the Kosena radish-derived fragments in the SW18 mitochondrial genome had sequences at their ends (ranging from 63 bp to 628 bp) that are identical to the sequences at the sites of insertion on the Westar rapeseed-derived mitochondrial genome. This suggests that these insertions were mediated by homologous recombination. These results confirm at the nucleotide level that a desired CMS-associated gene (orf125) along with a few extraneous genes from radish were successfully transferred.

A simple method was developed to detect signatures of ongoing selective sweeps in single nucleotide polymorphism (SNP) data. Based largely on the traditional site frequency spectrum (SFS), the method additionally incorporates linkage disequilibrium (LD) between pairs of SNP sites and uniquely represents both SFS and LD information as hierarchical “barcodes.” This barcode representation allows the identification of a hitchhiking genomic region surrounding a putative target site of positive selection, or a core site. Sweep signals at linked neutral sites are then measured by the proportion (F_c) of derived alleles within the hitchhiking region that are linked in the derived allele group defined at the core site. In measuring F_c or intra-allelic variability in an informative way, certain conditions for derived allele frequencies are required, as illustrated with the human ST8SIA2 locus. Coalescent simulators with and without positive selection are used to assess the false-positive and false-negative rates of the F_c statistic. To demonstrate its power, the method was further applied to the LCT, OCA2, EDAR, SLC24A5 and ASPM loci, which are known to have undergone positive selection in human populations. Overall, the method is powerful and can be used to identify core sites responsible for ongoing selective sweeps.

The Japanese rat snake (Elaphe climacophora) is a common species in Japan and is widely distributed across the Japanese islands. An albino mutant of the Japanese rat snake (“pet trade” albino) has been bred and traded by hobbyists for around two decades because of its remarkable light-yellowish coloration with red eyes, attributable to a lack of melanin. Another albino Japanese rat snake mutant found in a natural population of the Japanese rat snake at high frequency in Iwakuni City, Yamaguchi Prefecture is known as “Iwakuni no Shirohebi”. It has been conserved by the government as a natural monument. The Iwakuni albino also lacks melanin, having light-yellowish body coloration and red eyes. Albino mutants of several organisms have been studied, and mutation of the tyrosinase gene (TYR) is responsible for this phenotype. By determining the sequence of the TYR coding region of the pet trade albino, we identified a nonsense mutation in the second exon. Furthermore, RT-PCR revealed that TYR transcripts were not detected in this snake. These findings suggest that mutation of TYR is responsible for the albino phenotype of the pet trade line of the Japanese rat snake. However, the Iwakuni albino did not share this TYR mutation; thus, these two albino lines differ in their origins.