Evidence of tRNA cleavage in apicomplexan parasites: Half-tRNAs as new potential regulatory molecules of Toxoplasma gondii and Plasmodium berghei

Several lines of evidence demonstrated that organisms ranging from bacteria to higher animals possess a regulated endonucleolytic cleavage pathway producing half-tRNA fragments. In the present study, we investigated the occurrence of this phenomenon in two distantly related apicomplexan parasites, Toxoplasma gondii, the agent of toxoplasmosis, and the rodent malaria parasite Plasmodium berghei. A low-scale molecular characterization of the small RNA fraction of T. gondii revealed the endonucleolytic processing of 10 distinct tRNA species, with cleavage in the anticodon loop and upstream of the 3'-terminal CCA sequence yielding 5'- or 3'-end half-tRNAs. T. gondii and P. berghei exhibited variable rates of tRNA cleavage upon egress from host cells and in response to stage differentiation, amino acid starvation and heat-shock. Moreover, avirulent isolates of T. gondii and attenuated P. berghei parasites showed a higher rate of tRNA cleavage than virulent strains. Interestingly, half-tRNA production was significantly higher in the metabolically quiescent bradyzoite and sporozoite stages of T. gondii, compared to the fast-growing tachyzoite. Collectively, our findings shed light for the first time on the occurrence of tRNA cleavage in apicomplexan parasites and suggest a relationship between half-tRNA production and growth rate in this important group of organisms.