Polymorphic open reading frames encoding secretory proteins are located less than 3 kilobases from Theileria parva telomeres

Polymorphic, multicopy gene families are frequently located in subtelomeric regions of the genomes of parasitic protozoa. Theileria parva telomere-ass ociated (TA) DNA from two chromosomes contained long open reading frames (O RFs) 54% identical at the N-termini, whose 3' ends were 2670 and 2680 bp fr om the telomeric repeats. Probes derived from these ORFs revealed related s equences close to additional telomeres. The 3' end of an unrelated ORF was approximately 2720 bp from a third telomere. These are among the closest OR Fs to telomeres in any organism. Reverse transcription PCR detected transcr ipts originating within the telomeric multicopy gene family. Additional ORF s, with complex sequence similarities, were located centromeric to the telo mere-adjacent ORFs. Transcripts from the schizont stage of T. parva, contai ning domains with significant amino acid similarity to a 3529 codon ORF loc ated 6900 bp upstream of the telomeric repeats, were mapped to a subtelomer ic locus at a fourth telomere. Five telomeric ORFs contained predicted N-te rminal signal peptides and one of these signal peptides was functional in a heterologous system. Hybridisation data suggested extensive strain polymor phism between ORFs. Two of the telomere-adjacent ORFs were absent from the genome of a cloned T. parva parasite which can, nonetheless, be passaged th rough ricks and cattle. T. parva is unusual, among organisms so far studied , in the high density of potential coding sequences located directly adjace nt to telomeres and the apparent absence of extensive tracts of repeated se quences within the TA DNA. (C) 2000 Elsevier Science B.V. All rights reserv ed.