"A new study, published in Science Advances, reports the complete sequences for the two Arabidopsis NORs and how active and silent ribosomal RNA genes are distributed throughout the NORs. The paper was authored by postdoctoral researchers Dalen Fultz, Anastasia McKinlay and Ramya Enganti in the laboratory of Craig S. Pikaard, an Investigator of the Howard Hughes Medical Institute and a Distinguished Professor, and Carlos O. Miller Professor, in the Departments of Biology and Molecular and Cellular Biochemistry at Indiana University Bloomington (IUB). Previous studies by the lab had shown that active and silent ribosomal RNA genes subtypes co-exist but associated with different NORs, based on genetic tests. The new study identified more than 70 different gene subtypes, based on subtle differences, that are located either at NOR2 or NOR4, but not both. Knowing the physical positions of all of these subtypes, the authors conducted tests to determine whether each subtype was turned on, to make ribosomal RNA, or turned off. They also tested what happens in genetic mutants that are unable to silence their ribosomal RNA genes. What they found was that one NOR is nearly completely silenced in growing plants, whereas the other NOR accounts for almost all ribosomal RNA gene activity - but only in its central region. Regions of high gene activity were found to correlate with regions where chemical modification of the DNA (by addition of single carbon methyl groups) is low and where neighboring genes tend to be of the same subtype.
The results provide the first glimpse of how ribosomal RNA genes are organized and regulated in the context of complete NORs."