... some RFPs express as mixtures of proteins with either green or red chromophores. DsRed and its mutant mPlum are notable examples of RFPs that express in this fashion. Fast and complete maturation to the red chromophore is a desired property for RFPs used in imaging experiments. Here, we present a rational approach to enhance RFP chromophore maturation in the DsRed mutant mPlum. Because residues at position 16 interact directly with the peptide bond that oxidizes during red chromophore formation, we postulated that this interaction could interfere with the oxidation reaction at this position. Using rational design, we converted a yellow-emitting mPlum mutant to a red-emitting RFP by replacing Glu16 with small non-polar amino acids. We also created an optimized mPlum mutant (mPlum-E16P) that matures almost exclusively to the red chromophore.