Methionyl-tRNA Synthetase from Escherichia coli: Structure-Function Relationships of a Dimeric Enzyme with Repeated Sequences

Abstract

Aminoacyl-tRNA synthetases display a variety of molecular structures. For instance, in Escherichia coli the molecular weights of native aminoacyl-tRNA synthetases range from 44,000 in the case of cysteinyl-tRNA synthetase to 265,000 in the case of phenylalanyl-tRNA synthetase (Fayat et al. 1974). Moreover, depending on the enzyme considered, the synthetases consist of a single polypeptide chain or result from the assembly of identical or nonidentical subunits. Structures of the α, α₂, and α₂β₂ types can be distinguished (for a review, see Ofengand 1977). Until recently, this complex pattern of structures has defied any attempt to find a unifying trait within this class of isofunctional enzymes. In the past few years it has been shown that several aminoacyl-tRNA synthetases exhibit extensive sequence duplication inside polypeptide chains. Those enzymes that have so far been shown to possess this property are composed of one or more 100,000-m.w. polypeptide chain. On the basis of this observation, the bacterial aminoacyl-tRNA synthetases can be schematically grouped into a minimum of four nonexclusive classes based on protomer number and degree of sequence duplication.

The first class groups the enzymes composed of two identical subunits that do not have internal repeats. The molecular weights of the protomers are in the range of 35,000–55,000. The second class are monomeric enzymes of about 100,000 m.w. with extensive duplication (Kula 1973; Koch et al. 1974; Waterson and Konigsberg 1974; Bruton 1975). The third class is represented by the bacterial methionyl-tRNA synthetase. This enzyme is a dimer (2 × 85,000) composed of...