Published online 11 February 2004
Nucleic Acids Research, 2004, Vol. 32, No. 3 1018-1027
© 2004 Oxford University Press
Mycobacterium tuberculosis Rv2118c codes for a single-component homotetrameric m1A58 tRNA methyltransferase
Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore 560012, India, 1 Department of Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA and 2 Molecular and Structure Biology Division, Central Drug Research Institute, Lucknow 226 001, India
*To whom correspondence should be addressed. Tel: +1 617 253 4702; Fax: +1 617 252 1556; Email: bhandary{at}mit.edu
The authors wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors
Modified nucleosides in tRNAs play important roles in tRNA structure, biosynthesis and function, and serve as crucial determinants of bacterial growth and virulence. In the yeast Saccharomyces cerevisiae, mutants defective in N1-methylation of a highly conserved adenosine (A58) in the T
C loop of initiator tRNA are non-viable. The yeast m1A58 methyltransferase is a heterotetramer consisting of two different polypeptide chains, Gcd14p and Gcd10p. Interestingly, while m1A58 is not found in most eubacteria, the mycobacterial tRNAs have m1A58. Here, we report on the cloning, overexpression, purification and biochemical characterization of the Rv2118c gene-encoded protein (Rv2118p) from Mycobacterium tuberculosis, which is homologous to yeast Gcd14p. We show that Rv2118c codes for a protein of 
31 kDa. Activity assays, modified base analysis and primer extension experiments using reverse transcriptase reveal that Rv2118p is an S-adenosyl-L-methionine-dependent methyltransferase which carries out m1A58 modification in tRNAs, both in vivo and in vitro. Remarkably, when expressed in Escherichia coli, the enzyme methylates the endogenous E.coli initiator tRNA essentially quantitatively. Furthermore, unlike its eukaryotic counterpart, which is a heterotetramer, the mycobacterial enzyme is a homotetramer. Also, the presence of rT modification at position 54, which was found to inhibit the Tetrahymena pyriformis enzyme, does not affect the activity of Rv2118p. Thus, the mycobacterial m1A58 tRNA methyltransferase possesses distinct biochemical properties. We discuss aspects of the biological relevance of Rv2118p in M.tuberculosis, and its potential use as a drug target to control the growth of mycobacteria.
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