Sequence microheterogeneity of the three small subunit ribosomal RNA genes of Babesia bigemina: expression in erythrocyte culture.

Three distinct ribosomal RNA (rRNA) transcription units (rDNA units), designated A, B and C, were identified in the intraerythrocytic protozoan parasite, Babesia bigemina. These rDNA units were cloned, and restriction maps were constructed showing the approximate location of the small and large rRNA coding regions. The arrangement of the genes in the genome and copy number analysis suggests the presence of a single copy of each rDNA unit per haploid genome. The complete nucleotide sequence of the small subunit rRNA coding region (1693 bp) and parts of the 5' and 3' flanking regions were determined for all three units. Units A and B have identical sequences, but unit C differs from units A and B at ten nucleotide positions, two in the small subunit rRNA coding region and four each in the adjacent 5' and 3' flanking regions. The differences in the coding region are confirmed in genomic DNA and RNA from two different isolates of B.bigemina. The RNA of both sequence types is transcribed in parasites from erythrocyte culture, however, the products of gene units A + B accumulate at a ratio of approximately 4:1 compared with the product of unit C.