Small trypanosome RNA-binding proteins TbUBP1 and TbUBP2 influence expression of F-box protein mRNAs in bloodstream trypanosomes

In the African trypanosome Trypanosoma brucei nearly all control of gene expression is posttranscriptional; sequences in the 3'-untranslated regions of mRNAs determine the steady-state mRNA levels by regulation of RNA turnover. Here we investigate the roles of two related proteins, TbUBP1 and TbUBP2, containing a single RNA recognition motif, in trypanosome gene expression. TbUBP1 and TbUBP2 are in the cytoplasm and nucleus, comprise ca. 0.1% of the total protein, and are not associated with polysomes or RNA degradation enzymes. Overexpression of TbUBP2 upregulated the levels of several mRNAs potentially involved in cell division, including the CFB1 mRNA, which encodes a protein with a cyclin F-box domain. CFB1 regulation was mediated by the 3'-untranslated region and involved stabilization of the mRNA. Depletion of ThUBP2 and TbUBP1 inhibited growth and downregulated expression of the cyclin F box protein gene CFB2; trans splicing was unaffected. The results of pull-down assays indicated that all tested mRNAs were bound to TbUBP2 or TbUBP1, with some preference for CFBI. We suggest that TbUBP1 and TbUBP2 may be relatively nonspecific RNA-binding proteins and that specific effects of overexpression or depletion could depend on competition between various different proteins for RNA binding.