Distribution and expression of nucleolus organizers and ribosomal genes in man.

A study has been made of the distribution and expression of nucleolus organizers and ribosomal genes in man. Using filter hybridisation techniques, an inter-individual variation in the number of ribosomal cistrons has been found. This number is negatively correlated with the number of active nucleolus organizers (NORs) as determined by the ammoniacal silver staining (Ag-AS) technique. The Ag-AS technique in mammalian systems stains a protein component of the NORs that is, or has been active. Silver staining through the mitotic cycle reflects the nucleolar changes, and corresponding changes in activity of the NORs. Further studies using both the light and electron microscopes, reveal that each individual has a characteristic silver staining pattern that does not change with time. Evidence is presented that the activity of NORs in trisomy 21 patients may be disturbed, and this may be correlated with an increased frequency of satellite associations and an increased nucleolar coefficient in such patients. In situ hybridisation studies indicate that not necessarily only the largest NORs are active. While the specificity for active ribosomal cistrons in mammalian systems is high, this specificity is lost in some non-mammalian systems. Lampbrush chromosomes of Triturus cristatus carnifex did not have silver-positive chromosomal NORs, but extrachromosomal nucleoli were consistently silver-positive. In addition, a number of other sites proved to be silver-positive. One of these sites may be the location of the 5S RNA genes, but the function of the other loci is unknown. In Plethodon cinereus cinereus lampbrush chromosomes, there are many silver-positive extrachromosomal nucleoli, but a maximum of three silver-positive chromosomal structures A similar situation was found in polytene chromosomes of Chironomus thummi thummi, where the nucleolus and approximately 16 chromosomal bands were silver-positive. A band situated at, or close to, the site of the 5S RNA genes was consistently silver-positive.