A protein biosensor having increased signal intensity to sugar
concentration and the use thereof. A fluorescent indicator protein is
described, in which fluorescent proteins of different emission ranges are
fused to both ends of a binding protein undergoing a conformational
change due to binding of sugars, such that a change in the concentration
of various sugars involved in intracellular metabolism, e.g., maltose,
can be detected by measuring a change in emission intensities, caused by
FRET (fluorescence resonance energy transfer). A method for detecting a
change in the concentration of sugars using such fluorescent indicator
protein is also described. The fluorescent indicator protein has
excellent signal intensity enabling precision measurement of
intracellular concentration of various sugars. In one implementation,
high signal intensity fluorescent indicator proteins can be prepared from
proteins otherwise used as biosensors, to enable them to be more widely
useful.