Ribosomal RNA genes of Lilium henryi are almost completely methylated at CG and CNG sequences. A short under-methylated region was detected between 2.05 and 2.4 kbp upstream of the 18S sequences. It included the only sites of digestion by four methylation-sensitive restriction endonucleases - PstI, Hae II, Eco RII and Hpa II. Only about 15%-20% of rDNA repeats from shoot meristem are susceptible to each of the enzymes. The same repeats are apparently cut by all enzymes and occur in contiguous blocks. Because the region involved is likely to include regulatory sequences it may be that under-methylation occurs specifically in active rDNA repeats. To test this, rDNA was examined from pollen mother cells at pachytene where transcription has fallen to near zero. Under-methylation levels here were similar to those in shoot meristem tissue. Thus methylation of this region is not the agent responsible for rDNA gene inactivation in pachytene cells and it does not occur immediately genes become inactive. Even so, sequences in this region might be prevented from becoming methylated in transcribing repeats.

No abstract

We have examined the location and organization of nucleolar organizing regions (NORs) in the species Lilium henryi (with four NORs per diploid cell), L. longiflorum, and L. speciosum (each with six NORs), and the hybrids Lilium × 'Black Beauty' (five NORs) and Lilium × parkmannii (eight NORs). The relative number of genes in individual NORs was assayed by in situ hybridization using in vitro labelled ribosomal RNA (rRNA). An estimate of their relative transcriptional activity was obtained by scoring silver band size over the constriction. There was no clear correlation between gene number and activity at specific NORs. Rather, gene number correlated quite well with the relative size of heterochromatin usually found adjacent to NOR constrictions and stained by an acid-banding method. It is possible that many but a variable fraction of the rRNA genes in Lilium NORs are held inactive in nucleolar heterochromatin.Key words: ribosomal RNA genes, Lilium, nucleolus, in situ hybridization, heterochromatin, silver banding.