Abstract
Most eukaryotic ribosomes contain 26/28S, 5S, and 5.8S large subunit ribosomal RNAs (LSU rRNAs) in addition to the 18S rRNA of the small subunit (SSU rRNA). However, in kinetoplastids, a group of organisms that include medically important members of the genus Trypanosoma and Leishmania, the 26/28S large subunit ribosomal RNA is uniquely composed of 6 rRNA fragments. In addition, recent studies have shown the presence of expansion segments in the large ribosomal subunit (60S) of Trypanosoma brucei. Given these differences in structure, processing and assembly, T. brucei ribosomes may require biogenesis factors not found in other organisms. Here, we show that one of two putative 3-methylcytidine methyltransferases, TbMTase37 (a homolog of human methyltransferase-like 6, METTL6), is important for ribosome stability in T. brucei. TbMTase37 localizes to the nucleolus and depletion of the protein results in accumulation of ribosomal particles lacking srRNA 4 and reduced levels of polysome associated ribosomes. We also find that TbMTase37 plays a role in cytokinesis, as loss of the protein leads to multi-flagellated and multi-nucleated cells.
Highlights
Polycistronic transcript in the nucleolus by RNA polymerase I
The only major difference between these proteins is the absence of the N-terminal actin-binding domain found in S. cerevisiae; this domain is dispensable for methylation activity in yeast[16,17]
In this study we have characterized a putative methyltransferase from T. brucei and demonstrated its importance in the stability of the large ribosomal subunit and cell division
Summary
Polycistronic transcript in the nucleolus by RNA polymerase I. This transcript includes the 18S rRNA of the SSU and six of the seven parts of the LSU rRNA This polycistronic RNA is matured by a series of cleavage and post-transcriptional modification steps involving small nucleolar RNAs, various endo- and exonucleases and several modification enzymes. Detailed 80S ribosome biogenesis studies have been conducted in a few organisms including mammals, Xenopus and most extensively in the yeast Saccharomyces cerevisiae. These studies suggest that hundreds of proteins are involved in the biogenesis process to direct pre-rRNA processing, modify rRNA, shuttle assembling particles or chaperone assembly. Reduction in TbMTase[37] levels causes a defect in cytokinesis, suggesting a link between ribosome stability and/or function and cell division in T. brucei
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