An in vitor 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 condon for proline.