An upstream element of the TamS1 gene is a site of DNA-protein interactions during differentiation to the merozoite in Theileria annulata

Apicomplexan parasites are major pathogens of humans and domesticated anima ls. A fundamental aspect of apicomplexan biology, which may provide novel m olecular targets for parasite control, is the regulation of stage different iation. Studies carried out on Theileria annulata, a bovine apicomplexan pa rasite, have provided evidence that a stochastic process controls different iation from the macroschizont to the merozoite stage. It was postulated tha t this process involves the presence of regulators of merozoite gene expres sion in the preceding stage of the life cycle, and that during differentiat ion a quantitative increase of these factors occurs. This study was carried out to test these postulations. Nuclear run-on analysis showed that TamS1 expression is controlled, at least in part, at the transcriptional level. T he transcription start site showed homology with the consensus eukaryotic i nitiator motif, and study of the 5' upstream region by the electrophoretic mobility-shift assay demonstrated that a 23 bp motif specifically bound fac tors from parasite-enriched nuclear extracts. Three complexes were shown to bind to a 9 bp core binding site (5'-TTTGTAGGG-3'). Two of these complexes were present in macroschizont extracts but were found at elevated levels d uring differentiation. Both complexes contain a polypeptide of the same mol ecular mass and may be related via the formation of homodimer or heterodime r complexes, The third complex appears to be distinct and was detected at t ime points associated with the transition to high level merozoite gene expr ession.