Molecular and chromosomal organization of DNA sequences coding for the ribosomal RNAs in cereals

Analysis of the 5S RNA Pool in Arabidopsis thaliana: RNAs Are Heterogeneous and Only Two of the Genomic 5S Loci Produce Mature 5S RNA

“…These tandem arrays may be localized on one or several chromosomes and are separated from the genes encoding the large rRNAs (Long and Dawid, 1980;Appels et al 1980;Ellis et al 1988). It is assumed that only a few highly homologous 5S genes are transcriptionally active, whereas the majority of potentially active genes remain silent (Fulnecek et al 1998).…”

Chromosomal localization of ribosomal DNA sequences in an apple rootstock using a digoxygenin detection system

“…The nucleoli were of two size classes (Fig 3), reflecting differences in amount or rate of transcription at the rDNA loci on chromosome pairs 6 and 14, or difierent rates of assembly or export of ribosomes [ 26]． Onr finding that rDNA genes in Malus are localized on two pairs of chromosomes is consistent with earlier in situ hybridization for the diploid angiosperm species including A11ium cepa，A．fistulogum, AraNdopsis thaliana，Pisum sativum ，and Hordeum vulgare. In contrast in Secale cereale hybridization only one pair of chromosomes has been reported [4,5,13,24]．…”

“…[23]. Secondary constrictions which have been associated with plant NORs in some studies [4,5,10,16,24] were not observed． Hybridization of the rDNA sequence was detected at a 90％ frequency(over ten preparations) as patches of dark purplish-brown precipitate at the end of the short arms of chromosomes 6 and 14 (Fig 1b)，while control preparations(either DNase pretreatment of chromosomes or hybridization without a probe) failed to give such a colour reaction． Nucleolar organiser regions stained with silver were visible as tight black spots whereas non-specific staining was yellowish brown and tended to be more diffuse． The ten preparations examined indicated the presence of NORs at a frequency of 70％ for chromosome 6 and 90％for chromosome 14．NORs were present at the end of the short arms for both chromosome pairs． These results confirm the localization of ribosomal R N A genes detected with in situ hybridisation by the rDNA probe (Fig 2) since the acidic silver staining of ribonucleic protein accumulating around active NORs in interphase and remaining at mitosis is an indication of gene activity at rDNA sites during the preceding interphase [14]． When silver stained interphase preparations were screened to determine the maximum number of nucleoli and hence number of active rDNA loci，most interphase nuclei were found to contain one large nucleolus formed by complete fusion of nucleoli during the cell cycle，but few contained two, three, or the primary number of four [25], indicating that all four rDNA loci can be active as reported for AraNdopsis [13]. The nucleoli were of two size classes (Fig 3), reflecting differences in amount or rate of transcription at the rDNA loci on chromosome pairs 6 and 14, or difierent rates of assembly or export of ribosomes [ 26]． Onr finding that rDNA genes in Malus are localized on two pairs of chromosomes is consistent with earlier in situ hybridization for the diploid angiosperm species including A11ium cepa，A．fistulogum, AraNdopsis thaliana，Pisum sativum ，and Hordeum vulgare.…”

A cytogenetical and molecular analysis of the ribosomal cistrons of Allium sphaerocephalon L. (Liliaceae)

“…While there are species like rye (Appels et at., 1980) or maize (Givens & Phillips, 1976) in which the number of NOR is constant, there are, in contrast, other cases such as man (Ferraro & Prantera, 1988), mouse (Suzuki et a!., 1990), some species of salmonids (Phillips et al, 1988) or of the genus Alhium (Schubert & Wobus, 1985), in which the number of NOR varies between individuals or even between cells. Possible explanations for these variations have been attributed to mechanisms such as activation-inactivation (Ferraro & Prantera, 1988), duplication-deletion (Sato, 1981;Jamilena et al, 1990), unequal crossing-over (Suzuki et al, 1990), or even transpositional phenomena (Schubert & Wobus, 1985).…”