Evidence for translational control of beta-tubulin synthesis during differentiation of Leishmania donovani

Tubulin biosynthesis was rapidly induced during transformation of the mammalian (amastigote) stage of the kinetoplastid parasite Leishmania donovani to flagellated promastigotes. However, transcription of beta-tubulin genes occurred constitutively, as judged by nascent RNA synthesis in isolated nuclei and Northern blotting of steady-state mRNA. Two mRNA species of 2.2 and 2.4 kb were shared by the two cell-types, while a third 2.6 kb species, constituting about 20% of the total, was present in large amounts in promastigotes. RNase protection experiments demonstrated sequence microheterogeneity in the 5'-untranslated region, the pattern of which was identical in promastigotes and amastigotes. By primer extension assays, heterogeneity in the 5'-terminal cap structure of amastigote beta-tubulin mRNA and differential pausing of reverse transcriptase within the mini-exon leader region were detected. These differences correlated with enhanced translational efficiency of tubulin mRNA from promastigotes in a rabbit reticulocyte lysate system. The results indicate that translational control plays a major role in tubulin induction during L. donovani differentiation.