A 12 year old boy has a productive cough characterized by large volumes of foul-smelling sputum. Three years ago, the patient was diagnosed with pancreatic insufficiency, as evidenced by repetitive gastrointestinal symptoms of steatorrhea. After culture of the sputum, colorless, oxidase-positive colonies with a fruity aroma develop on the agar. The function of which of the following proteins is most likely inhibited by the bacteria responsible for this boy's infection?