Background:

Streptavidin is a biotin binding protein present in the fermentation broth of the bacterium Streptomyces avidinii. Each molecule of streptavidin can bind four molecules of biotin with a high affinity constant (K_d~10 ^-15). Unlike native avidin, streptavidin is not glycosylated and has a near neutral isoelectric point (pI ~ 5-6) vs a pI of 10 for native avidin.

R-phycoerythrin (R-PE) has a molecular weight of 240 kDa and is purified from red algae. R-PE is an intensely bright phycobiliprotein with an absorption maxima at approximately 496, 546 and 565 nm, and a fluorescence emission maximum of ~578 nm.