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Mitotic movement of mystery methylase
One of the mechanisms that contributes to the regulation of gene activity is DNA methylation at cytosine residues in CpG dinucleotides. These methylated sites may be maintained through DNA replication by methylation of the complementary GpC in the daughter strand, catalysed by type 1 methyltransferases (MET1). Kim et al. (Nara, Japan, pp. 845–856) have investigated MET1 in tobacco (Nicotiana tabacum). They obtained the cDNA encoding the enzyme and used a baculovirus vector to express it in an insect cell line. The purified enzyme was assayed with both hemi-methylated and unmethylated DNA but showed no methylase activity with either target. The reasons for this are not clear but it is possible that the folding of the over-expressed protein resulted in the active site being masked by intramolecular interactions. In order to find out whether the protein expressed from cDNA was active in planta, the authors over-expressed it in tobacco. Surprisingly, the plants over-expressing the enzyme showed less DNA methylation than control plants. This was not caused by multi-copy suppression of gene expression. Rather it seems that the presence of excess enzyme suppressed the activity of that expressed from the plant’s own genome. Nevertheless, the location of MET1 during the cell cycle is consistent with it being involved in some way with DNA. It is located in the nucleus during ‘interphase’, including S-phase, but is present mainly in the cytoplasm during mitosis, especially metaphase. This pattern is similar to that exhibited by Ran GTPase, with which MET1 was shown to interact physically. In a previous paper (Yano et al., Annals of Botany 98: 1179–1187), the same research group showed that Ran GTPase interacted with a methyl-CpG-binding protein and with that protein was implicated in chromatin movement. It is thus possible that MET1, in addition to its still unconfirmed methylase activity, also has a role in the dynamic behaviour of chromatin.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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