RESCUING AN ESSENTIAL ENZYME RNA COMPLEX WITH A NONESSENTIAL APPENDEDDOMAIN

Certain protein-RNA complexes, such as synthetase-tRNA complexes, are essential for cell survival, These complexes are formed with a precise molecular fit along the interface of the reacting partners, and mutat ional analyses have shown that amino acid or nucleotide substitutions at the interface can be used to disrupt functional or repair non-funct ional complexes, In contrast, we demonstrate here a feature of a eukar yote system that rescues a disrupted complex without directly re-engin eering the interface, The monomeric yeast Saccharomyces cerevisiae glu taminyl-tRNA synthetase, like several other class I eukaryote tRNA syn thetases, has an active-site-containing 'body' that is closely homolog ous to its Escherichia coli relative, but is tagged at its N-terminus with a novel and dispensable appended domain whose role has been obscu re, Because of differences between the yeast and E.coli glutamine tRNA s that presumably perturb the enzyme-tRNA interface, E.coli glutaminyl -tRNA synthetase does not charge yeast tRNA, However, linking the nove l appended domain of the yeast to the E.coli enzyme enabled the E.coli protein to function as a yeast enzyme, in vitro and in vivo. The appe nded domain appears to contribute an RNA interaction that compensates for weak or poor complex formation, In eukaryotes, extra appended doma ins occur frequently in these proteins, These domains may be essential when there are conditions that would otherwise weaken or disrupt form ation of a critical RNA-protein complex. They may also be adapted for other, specialized RNA-related functions in specific instances.