Synthesis of 5S rRNA and putative precursor tRNAs in nuclei isolated from wheat embryos

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-HpaII on this binding. We have found that a number of NP specifically bind to the rDNA promoter, and to upstream and downstream IGS sequences. A 240-kDa NP binds specifically to a 174-bp fragment located downstream from the transcription start point (tsp), and this fragment competes with a 135-bp subrepeat element fragment for specific binding to this 240-kDa NP. Specific binding of the 240-kDa NP to the 174-bp fragment is greatly inhibited by methylation of internal cytosine residues in its two CCGG sites, whereas NP binding to the 135-bp subrepeat element is not affected by M-HpaII methylation. A 202-bp fragment containing the tsp is partially 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 interference with the binding of some of these transcription factors to their respective regulatory elements.

Externally added DNA molecules support initiation of transcription in isolated nuclei from petunia

Nuclei isolated from protoplasts transfected with the pUC8CaMVCAT and pDO432 plasmids were able to support, in run off experiments, the synthesis of specific transcripts as was evident from analysis by dot blot hybridization. Also the addition of the above plasmids to nuclei, prepared from non-transfected protoplasts, supported the synthesis of specific transcripts. Dot blot analysis showed that most of the transcripts obtained were complementary to the relevant gene sequences. alpha-Amanitin, at concentrations which are known to block the activity of RNA polymerase II, significantly inhibited the synthesis of specific transcripts by the isolated nuclei. The transcription activity was found to be predominantly associated with the nuclear fraction while the transcription products (RNA molecules) appeared in the supernatant obtained following sedimentation of the nuclei.

Specific transcription and reinitiation of class III genes in wheat embryo nuclei and chromatin

Chromatin isolated from wheat germ embryos has a transcription efficiency for RNA polymerase III (pol III) closely approaching that for isolated wheat germ nuclei. Transcription in nuclei and chromatin is inhibited 5-10-fold by the addition of heparin, suggesting that free pol III molecules bind to chromatin and initiate transcription during thein vitro incubation. Nuclei were shown to have similar transcriptional activity in potassium chloride and potassium acetate. Nuclei and chromatin exhibited different salt optima for transcription. Neither nuclei nor chromatin were strongly stimulated by exogenous protein fractions. The data presented here suggest that in wheat germ nuclei the complete transcriptional apparatus is stably bound to the chromatin. Wheat germ nuclei may serve therefore as an enriched source for a solublein vitro transcription system.

DNase I sensitivity of ribosomal RNA genes in chromatin and nucleolar dominance in wheat

Ribosomal RNA genes at different nucleolar organizer (NOR) loci in hexaploid wheat are expressed at different levels. The degree of expression of a particular organizer depends on the genetic background, especially on the presence of other NOR loci. For example, when chromosome 1U of Aegilops umbellulata is introduced into the hexaploid wheat cultivar "Chinese Spring" the A. umbellulata NOR accounts for most of the nucleolar activity and seems to suppress the activity of the wheat NOR loci. Even in wild-type "Chinese Spring", the NOR on chromosome 1B is partially dominant to that on chromosome 6B, since the 1B locus is more active in spite of having fewer genes. We have previously shown that these and other examples of nucleolar dominance in wheat are associated with undermethylation of cytosine residues in certain regions of the dominant rDNA. Here, we show that rRNA genes at dominant loci are organized in a chromatin conformation that renders them more sensitive to DNase I digestion than other rRNA genes. In addition, we have mapped several DNase I-hypersensitive sites in the intergenic spacer region of the rDNA repeating unit. Some of these sites are located near the initiation region for the 45 S rRNA precursor, while others are associated with a series of short direct repeats 5' to the 45 S rRNA initiation site. The results are discussed in terms of a model in which repeated sequences in the wheat intergenic DNA are presumed to function as upstream promoters and transcriptional enhancers similar to those in Xenopus.