The large form of ferredoxin-NADP reductase (FNR) was treated with 66%iso-propyl alcohol and fractionated. The precipitate contained the small form of FNR, which was incapable of reassociating to the large form. The supernatant contained a new protein factor of low molecular weight. When the protein factor was isolated from the supernatant and added to the small form of FNR, the large form of FNR was reconstituted under high salt conditions. Experimental findings indicate that the large form of FNR was composed of two molecules of the small form of FNR which were connected by a protein factor. The protein factor was purified by hydrophobic interaction column chromatography using butyl-Toyopearl 650M and its molecular weight was determined to be about 10,000 by gel filtration. It was a colorless protein with an unusual absorption spectrum in ultra-violet regions. The protein factor was very stable against heat but was digested by trypsin. It was named“Connectein”after its connective act
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