GENERAL STRUCTURE FUNCTION PROPERTIES OF MICROBIAL METHIONYL-TRANSFER-RNA SYNTHETASES/

Alignment of the sequences of methionyl-tRNA synthetases from various microbial sources shows low levels of identities. However, sequence id entities are clustered in a limited number of sites, most of which con tain peptide patterns known to support the activity of the Escherichia coli enzyme. In the present study, site-directed mutagenesis was used to probe the role of these conserved residues in the case of the Baci llus stearothermophilus methionyl-tRNA synthetase. The B. stearothermo philus enzyme was chosen in this study because it can be produced as a n active truncated monomeric form, similar to the monomeric derivative of E. coli methionyl-tRNA synthetase produced by mild proteolysis. Th e two core enzyme molecules share only 27% identical residues. The res ults allowed the identification of the binding sites for ATP, methioni ne and tRNA, as well as that responsible for the tight binding of the zinc ion to the enzyme. It is concluded that the thermostable syntheta se adopts a three-dimensional folding very similar to that of the E. c oli one. Therefore, the two methionyl-tRNA synthetase sequences, altho ugh significantly different, maintain a common scafold with the functi onally important residues exposed at constant positions. Sequence alig nments suggest that the above conclusion can be generalized to the kno wn methionyl-tRNA synthetases from various sources.