Tandemly repeated hexanucleotide at Tetrahymena rDNA free end is generated from a single copy during development

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Tetrahymena ribosomal DNA (rDNA) exists as linear extrachromosomal molecules in the macronucleus.
• These extrachromosomal rDNA molecules possess tandemly repeated hexanucleotide sequences (CCCAAA) at their free ends.

Purpose of the Study:

• To investigate the origin of the tandemly repetitive hexanucleotide sequences at the free ends of extrachromosomal rDNA in Tetrahymena.
• To determine if these repetitive sequences are an inherited feature of the rDNA locus.

Main Methods:

• Nucleotide sequencing of the 3' end of the integrated micronuclear rDNA gene.
• DNA cloning and Southern hybridization to compare genomic DNA with cloned fragments.

Main Results:

• The single, integrated micronuclear rDNA gene contains only one CCCAAA sequence at the 3' end, unlike the 20-70 repeats found in the extrachromosomal rDNA.
• Southern hybridization confirmed the difference between the integrated and extrachromosomal rDNA sequences.
• Results were consistent across independently isolated DNA clones, ruling out cloning artifacts.

Conclusions:

• The tandemly repetitive hexanucleotide sequences at the free ends of extrachromosomal rDNA are not an inherited feature.
• These repetitive sequences are generated *de novo* during the development of the macronucleus in Tetrahymena.