Synthesis of Escherichia coli carbamoylphosphate synthetase initiates at a UUG codon

The ribosome binding region of the messenger RNA for the Escherichia coli carA gene contains two adjacent putative translational start codons, UUG and AUU, both of them unusual. By Edman degradation and mass spectrometry of purified carA protein, we show that only UUG is used in vivo. Translation initiation at UUG in carA appears about half as efficient as at AUG in lacZ.