Expression of ZmMET1, a gene encoding a DNA methyltransferase from maize, is associated not only with DNA replication in actively proliferating cells, but also with altered DNA methylation status in cold-stressed quiescent cells

Abstract

A cDNA fragment encoding part of a DNA methyltransferase was isolated from maize. The putative amino acid sequence identically matched that deduced from a genomic sequence in the database (accession no. AF063403), and the corresponding gene was designated as ZmMET1. Bacterially expressed ZmMET1 actively methylated DNA in vitro. Transcripts of ZmMET1 could be shown to exclusively accumulate in actively proliferating cells of the meristems of mesocotyls and root apices, suggesting ZmMET1 expression to be associated with DNA replication. This was confirmed by simultaneous decrease of transcripts of ZmMET1 and histone H3, a marker for DNA replication, in seedlings exposed to wounding, desiccation and salinity, all of which suppress cell division. Cold stress also depressed both transcripts in root tissues. In contrast, however, accumulation of ZmMET1 transcripts in shoot mesocotyls was not affected by cold stress, whereas those for H3 sharply decreased. Such a differential accumulation of ZmMET1 transcripts was consistent with ZmMET1 protein levels as revealed by western blotting. Expression of ZmMET1 is thus coexistent, but not completely dependent on DNA replication. Southern hybridization analysis with a methylation-sensitive restriction enzyme revealed that cold treatment induced demethylation of DNA in the Ac/Ds transposon region, but not in other genes, and that such demethylation primarily occurred in roots. These results suggested that the methylation level was decreased selectively by cold treatment, and that ZmMET1 may, at least partly, prevent such demethylation.

A chloroplast-resident DNA methyltransferase is responsible for hypermethylation of chloroplast genes in Chlamydomonas maternal gametes

“…The gene was therefore designated as CrMET1 (C. reinhardtii DNA methyltransferase 1). Sequence comparison of the catalytic domain with known methyltransferases indicated CrMET1 to resemble ZmMET1 from maize (15). To estimate its enzymatic activity, a 2-kb fragment of CrMET1 encoding the C-terminal region with a 68-kDa polypeptide was expressed in insect cells by using a baculo-system, in which the maximal size of proteins to be expressed is around 120 kDa.…”

Epigenetic control of plant stress response

“…Demethylation of one of the genes, NtAlix1, also occurs under the viral infection, showing that the induction of this gene under natural stress requires sequence demethylation. Steward et al [2000] also showed that transcriptional activation of ZmMI1 gene in maize seedlings was dependent on the cold stress-mediated sequence demethylation. ZmMI1 gene contains a retrotransposon-like sequence, and its activation is mirrored by the cold-induced rootspecific demethylation of Ac/Ds transposon regions followed by their activation [Steward et al, 2000].…”

“…Exposure to cold of root tissues of maize seedlings resulted in DNA demethylation of the nucleosome core regions [Steward et al, 2000]. In fact, the DNA replication was strongly reduced in chilled tissues, thus allowing speculations that genome hypomethylation was the result of active rather than passive demethylation.…”

Physiological functions of plant DNA methyltransferases

“…The second family contains chromomethylase (CMT) and the third features the domains rearranged methyltransferase (DRM). These enzymes have already been identified from a variety of plants, including maize, tobacco and rice (Finnegan and Denis 1993;Genger et al 1999;Olhoft 1998;Steward et al 2000;Nakano et al 2000;Bernacchia et al 1998;Pradhan et al 1998;Henikoff and Comai 1998;Rose et al 1998;Lindroth et al 2001;Bartee et al 2001;Tompa et al 2002;Papa et al 2001;Cao et al 2000;Wada et al 2003) (Table 2). In the following sections, I briefly summarize their properties and also findings with mutants for chromatin remodeling factors, which govern the global methylation status created by these enzymes.…”