An in vitro translation system can synthesize peptides in all three reading frames of the RNA template. When 5' - UCUCUCUC-(UC)_n-UCUCUCUC - 3' was used as the template in this in vitro translation system, the synthesized peptides contained 50% each of serine and leucine. When 5' - CCUCCUCCU-(CCU)_n-CCUCCU - 3' was used as the template, the synthesized peptides contained 33.3% each of serine, leucine, and proline. Deduce the codon for proline.