Arrangement of coding and non-coding sequences in the DNA molecules coding for rRNAs in Oxytricha sp.

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Abstract
All of the genes in the macronucleus of Oxytricha sp. occur on physically separate “gene-sized” DNA molecules. We have inserted the DNA molecule that codes for rRNA into a bacterial plasmid in order to study its structure and function. Using restriction nuclease mapping and hybridization of 125I-rRNAs to gel separated DNA fragments we have determined that the intact rDNA is 8,140±50 base pairs (bp) in length. Reading from one end, the molecule consists of ∼1,540 bp of non-coding DNA, ∼2,100±50 bp that code for 19S rRNA, ∼ 3,700±50 bp that code for 25S rRNA, and ∼620±50 bp of non-coding DNA. The 5.8S rRNA coding sequence (∼150 bp) occurs at one end of the 25S RNA coding region but which end is not known yet. All three rRNAs are encoded in the same strand of the DNA molecule, and transcription is in the order: 19S→25S.