<P> The synthesis of histidine in E. coli is a complex pathway involving 10 reactions and 10 enzymes . Synthesis begins with 5 - phosphoribosyl - pyrophosphate (PRPP) and finishes with histidine and occurs through the reactions of the following enzymes: </P> <P> HisG -> HisE / HisI -> HisA -> HisH -> HisF -> HisB -> HisC -> HisB -> HisD (HisE / I and HisB are both bifunctional enzymes) </P> <P> All of the enzymes are coded for on the his operon . This operon has a distinct block of the leader sequence, called block 1: </P> <P> Met - Thr - Arg - Val - Gln - Phe - Lys - His - His - His - His - His - His - His - Pro-Asp </P>

Where does the cell get its amino acid from