An Enhanced Mutant of Red Fluorescent Protein DsRed for Double Labeling and Developmental Timer of Neural Fiber Bundle Formation*

We developed a new variant of coral-derived red fluo-rescent protein, DsRed S197Y, which is brighter and es-sentially free from secondary fluorescence peak. This makes it an ideal reporter for double labeling with green fluorescent protein (GFP). Though purified protein shows only 20 % stronger fluorescence emission, culture cells that express DsRed S197Y exhibit a 3–3.5 times higher level of fluorescence than the cells that express wild-type DsRed. The much slower fluorescence maturation of DsRed than that of GFP is a beneficial feature for a fluo-rescent developmental timer application. When GFP and DsRed S197Y are expressed simultaneously, emissions start at different latency. This provides information about the time after the onset of expression. It reflects the order of cell differentiation if the expression is activated upo