Microsequence heterogeneity and expression of the LSU rRNA genes within the two single copy ribosomal transcription units of Theileria parva

The nucleotide sequences of the large subunit ribosomal RNA coding genes wi thin the two single copy ribosomal DNA transcription units of a cloned Thei leria parva isolate from a buffalo were determined. The two LSU rRNA coding units differed by 11 nucleotide substitutions and two deletions of 1 and 6 bp, all located in the 5' end of the LSU coding region. We also observed m icrosequence heterogeneity between the two buffalo parasite LSU sequences a nd the previously determined LSU rRNA gene of a T. parva parasite isolated from cattle. At all positions which were variable between the two LSU rRNA coding sequences of the buffalo-derived parasite, either unit 1 or unit 2 m atched the LSU rRNA coding sequence of the cattle-derived T. parva parasite in a mosaic pattern. Synthetic oligonucleotides specific for LSU units I a nd 2 of the buffalo T. parva were developed, and used to assay expression o f the two units. Both units were expressed in the intra-lymphocytic schizon t stage of T. parva. A 2.5-10-fold excess of rRNA derived from LSU unit 1, compared with unit 2, was observed in the schizont stage, the difference be ing attributable to variation in the level of expression of unit 2. Theiler ia represents the third genus of Sporozoan protozoa, in addition to Plasmod ium and Babesia, exhibiting rRNA coding genes, which are divergent in seque nce between different transcription units. (C) 2000 Elsevier Science B.V. A ll rights reserved.