title: Synthesis of signals for de novo DNA methylation in Neurospora crassa.
abstract: Most 5-methylcytosine in Neurospora crassa occurs in A:T-rich sequences high in TpA dinucleotides, hallmarks of repeat-induced point mutation. To investigate how such sequences induce methylation, we developed a sensitive in vivo system. Tests of various 25- to 100-bp synthetic DNA sequences revealed that both T and A residues were required on a given strand to induce appreciable methylation. Segments composed of (TAAA)(n) or (TTAA)(n) were the most potent signals; 25-mers induced robust methylation at the special test site, and a 75-mer induced methylation elsewhere. G:C base pairs inhibited methylation, and cytosines 5' of ApT dinucleotides were particularly inhibitory. Weak signals could be strengthened by extending their lengths. A:T tracts as short as two were found to cooperate to induce methylation. Distamycin, which, like the AT-hook DNA binding motif found in proteins such as mammalian HMG-I, binds to the minor groove of A:T-rich sequences, suppressed DNA methylation and gene silencing. We also found a correlation between the strength of methylation signals and their binding to an AT-hook protein (HMG-I) and to activities in a Neurospora extract. We propose that de novo DNA methylation in Neurospora cells is triggered by cooperative recognition of the minor groove of multiple short A:T tracts. Similarities between sequences subjected to repeat-induced point mutation in Neurospora crassa and A:T-rich repeated sequences in heterochromatin in other organisms suggest that related mechanisms control silent chromatin in fungi, plants, and animals.
authors.affiliation: Department of Biology and Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403-1229, USA.
authors.forename: Hisashi
authors.initials: H
authors.lastname: Tamaru
authors.forename: Eric U
authors.initials: EU
authors.lastname: Selker
mesh_terms: D020856:AT Rich Sequence / Q000502:physiology; D024761:AT-Hook Motifs / Q000502:physiology; D020029:Base Pairing / Q000502:physiology; D001483:Base Sequence; D001681:Biological Assay; D003596:Cytosine / Q000378:metabolism; D019175:DNA Methylation; D004271:DNA, Fungal / Q000235:genetics / Q000378:metabolism*; D004274:DNA, Recombinant / Q000502:physiology; D024202:Electrophoretic Mobility Shift Assay; D024243:HMGB1 Protein / Q000378:metabolism; D008969:Molecular Sequence Data; D009492:Neurospora crassa; D017354:Point Mutation; D012091:Repetitive Sequences, Nucleic Acid / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D013329:Structure-Activity Relationship
chemical_list: D004271:DNA, Fungal; D004274:DNA, Recombinant; D024243:HMGB1 Protein; D003596:Cytosine