This study by W.L. Gerlach and J.R. Bedbrook focuses on the cloning and characterization of ribosomal RNA (rRNA) genes from wheat and barley. The authors isolated DNA enriched for rRNA genes from actinomycin D-CsCl gradients and used it to clone ribosomal repeating units into the plasmid pAC184. They found that all five chimeric plasmids containing wheat rDNA and eleven of the thirteen containing barley rDNA were stable and included full-length ribosomal repeating units. Physical maps of these variants were constructed using various restriction endonucleases, revealing length variations in the repeat units due to differences in spacer regions. The study also compared Hae III and Hpa II digestion patterns of cereal rDNAs and cloned repeats, suggesting that most methylated cytosines in natural rDNA are in the -CpG- context. Incomplete methylation was observed at specific Bam HI sites in barley DNA. Additionally, detectable quantities of ribosomal spacer sequences were found only at genomic locations associated with ribosomal RNA gene repeats. The results provide insights into the organization and methylation patterns of rRNA genes in cereal plants.This study by W.L. Gerlach and J.R. Bedbrook focuses on the cloning and characterization of ribosomal RNA (rRNA) genes from wheat and barley. The authors isolated DNA enriched for rRNA genes from actinomycin D-CsCl gradients and used it to clone ribosomal repeating units into the plasmid pAC184. They found that all five chimeric plasmids containing wheat rDNA and eleven of the thirteen containing barley rDNA were stable and included full-length ribosomal repeating units. Physical maps of these variants were constructed using various restriction endonucleases, revealing length variations in the repeat units due to differences in spacer regions. The study also compared Hae III and Hpa II digestion patterns of cereal rDNAs and cloned repeats, suggesting that most methylated cytosines in natural rDNA are in the -CpG- context. Incomplete methylation was observed at specific Bam HI sites in barley DNA. Additionally, detectable quantities of ribosomal spacer sequences were found only at genomic locations associated with ribosomal RNA gene repeats. The results provide insights into the organization and methylation patterns of rRNA genes in cereal plants.