Constitutive expression of small subunit ribosomal RNA transcripts in Cryptosporidium parvum oocysts and intracellular stages

As in other apicomplexan protozoa. the ribosomal genes of Cryptosporidium p arvum are present in small number and dispersed on several chromosomes. Two types of ribosomal small subunit genes have been identified in bovine isol ates by sequencing and sequence-specific polymerase chain reaction. Using d enaturing gradient gel electrophoresis, ribosomal gene expression was inves tigated in C. parvum oocysts and intracellular stages grown in tissue cultu re. In a calf-propageted C. parvum, isolate, extra- and intracellular stage s expressed 2 types of transcripts. This observation contrasts with the dif ferential expression of rRNA genes in malar ia species. Analysis of ribosom al gene expression in oocysts of a different genotype indicates that some C . parvum isolates express only 1 type of small subunit ribosomal RNA. These observations highlight fundamental differences in ribosomal gene expressio n among apicomplexan protozoa and within the species C. parvum and support the view that differential expression in malaria is an adaptation to a dixe neous life cycle.