An intervening sequence in the gene coding for 25S ribosomal RNA of Tetrahymena pigmentosa

An intervening sequence of approximately 400 bp has been discovered in the 25S ribosomal RNA gene (rDNA) from macronuclei of Tetrahymena pigmentosa strain 6UM. Another strain, 8ALP, lacks the intervening sequence in its macronuclear rDNA. The presence of the intervening sequence was first suggested by a comparison of the restriction endonuclease maps of the rDNA from each strain. A region which differed in size between the two strains was shown to lie within the region coding for 25S ribosomal RNA by hybridization of the restriction fragments to labeled 25S rRNA. Direct visual evidence for the existence of the intervening sequence in 6UM and its absence from 8ALP was obtained by electron microscopy of R loop hybrids formed between the rDNA and 25S rRNA. The size and position of the intervening sequence within the gene were further defined by detailed restriction analyses of cloned fragments of the rDNA from each strain. The rDNA within the macronucleus of each of these strains is homogeneous, and thus the intervening sequence in the rDNA of strain 6UM must not interfere with transcription of the gene. Furthermore, the presence or absence of the intervening sequence has no obvious functional importance, since both strains can survive and grow equally well.