DNA topoisomerase IIIβ promotes cyst generation by inducing cyst wall protein gene expression in Giardia lamblia

Giardia lamblia causes waterborne diarrhoea by transmission of infective cysts. Three cyst wall proteins are highly expressed in a concerted manner during encystation of trophozoites into cysts. However, their gene regulatory mechanism is still largely unknown. DNA topoisomerases control topological homeostasis of genomic DNA during replication, transcription and chromosome segregation. They are involved in a variety of cellular processes including cell cycle, cell proliferation and differentiation, so they may be valuable drug targets. Giardia lamblia possesses a type IA DNA topoisomerase (TOP3 beta) with similarity to the mammalian topoisomerase III beta. We found that TOP3 beta was upregulated during encystation and it possessed DNA-binding and cleavage activity. TOP3 beta can bind to the cwp promoters in vivo using norfloxacin-mediated topoisomerase immunoprecipitation assays. We also found TOP3 beta can interact with MYB2, a transcription factor involved in the coordinate expression of cwp1-3 genes during encystation. Interestingly, overexpression of TOP3 beta increased expression of cwp1-3 and myb2 genes and cyst formation. Microarray analysis confirmed upregulation of cwp1-3 and myb2 genes by TOP3 beta. Mutation of the catalytically important Tyr residue, deletion of C-terminal zinc ribbon domain or further deletion of partial catalytic core domain reduced the levels of cleavage activity, cwp1-3 and myb2 gene expression, and cyst formation. Interestingly, some of these mutant proteins were mis-localized to cytoplasm. Using a CRISPR/Cas9 system for targeted disruption of top3 beta gene, we found a significant decrease in cwp1-3 and myb2 gene expression and cyst number. Our results suggest that TOP3 beta may be functionally conserved, and involved in inducing Giardia cyst formation.