Organization of the 5S RNA genes in macro- and micronuclei of Tetrahymena pyriformis.

The organization of the 5S genes in macro- and micronuclei of Tetrahymena pyriformis was studied using restriction endonucleases. After complete digestion of macronuclear DNA with BamH-I or Hpa I, 5S RNA hybridized to a DNA fragment of approximately 280 base pairs (bp). When macronuclear DNA was only partially digested with these enzymes, hybridization with 32P-5S RNA demonstrated an oligomeric series with a spacing of 280 bp. These results indicate that the 5S genes are tandemly repeated in macronuclei and that the repeating unit is 280 bp (or 180,000 daltons). Since 5S RNA is 120 nucleotides, we conclude that the 5S repeat units contain a 120 bp transcribed region and a 160 bp spacer region. When macronuclear DNA was digested with Eco RI, Bgl I, or Eco RI + Bgl I, 5S RNA hybridized to DNA of molecular weight 3--4 X 10(6), suggesting that these enzymes do not cleave within a 5S repeat. These 3--4 X 10(6) dalton fragments define the maximum size of an average cluster of 5S repeated units. Assuming the size of the 5S repeat to be 0.18 X 10(6) daltons, there are about 15--20 5S repeats per average tanden cluster, and since there are 350 5S-genes per haploid genome, there must be approximately 15--20 tandem arrays. Results obtained using micronuclear DNA suggest that organization of the 5S-genes is very similar in macro- and micronuclei. Macronuclear rRNA genes are extrachromosomal palindromic dimers. In contrast, 5S genes in Tetrahymena were found to be integrated within the genomes of both macro- and micronuclei and not linked to the rRNA genes. Moreover, it is unlikely that they are palindromes; rather they appear to be tandemly repeated in "head-to-tail" linkages. Thus the organization of the 5S genes in Tetrahymena is similar to that of higher eukaryotes.