Complementation in vitro of Phenylalanyl‐tRNA Synthetases of Escherichia coli

Complementation in vitro has been studied between the phenylalanyl‐tRNA synthetase subunits present in cell‐free extracts of temperature‐sensitive mutants of Escherichia coli . These mutant subunits, which judged from their size are dimers, form a hybrid enzyme with the dissociation products present in urea treated wild type enzyme preparations. The enzymatically active complementation product possesses the same size as the native enzyme but differs from it in its higher heat sensitivity. Complementation between the phenylalanyl‐tRNA synthetase cross‐reacting material of the different mutants alone could not be demonstrated under the conditions tested.