METHYLATION-DEPENDENT BINDING OF WHEAT NUCLEAR PROTEINS TO THE PROMOTER REGION OF RIBOSOMAL-RNA GENES

Here, we report data on the binding of wheat nuclear proteins (NP) to the promoter region of the rDNA intergenic spacer (IGS), with emphasis on the possible effects of methylation with M . HpnII on this binding . We have found that a number of NP specifically bind to the rDNA prom oter, and to upstream and downstream IGS sequences. A 240-kDa NP binds specifically to a 174-bp fragment located downstream from the transcr iption start point (tsp), and this fragment competes with a 135-bp sub repeat element fragment for specific binding to this 240-kDa NP. Speci fic binding of the 240-kDa NP to the 174-bp fragment is greatly inhibi ted by methylation of internal cytosine residues in its two CCGG sites , whereas NP binding to the 135-bp subrepeat element is not affected b y M . HpaII methylation. A 202-bp fragment containing the tsp is parti ally homologous to the 174-bp downstream region fragment. Nevertheless , it neither binds the 240-kDa NP nor is its binding to NP affected by methylation of CCGG sites. Thus, multiple specific NP may be involved in rDNA transcription initiation. Methylation-dependent inhibition of rDNA transcription initiation appears to be mediated by direct interf erence with the binding of some of these transcription factors to thei r respective regulatory elements.