Identification of a periplasmic C-type cytochrome as electron donor to the plasma membrane-bound cytochrome oxidase of the cyanobacterium Nostoc Mac.

Photoautotrophically grown cyanobacterium Nostoc sp. strain Mac (PCC 8009) released up to about 10 nmol of a c-type cytochrome per ml packed cells after treatment with EDTA under conditions that left the plasma membrane absolutely intact as judged from the absence of cytosolic proteins in the supernatant. Spectra of the ascorbate reduced cytochrome revealed peaks at 553, 522 and 416 nm. The protein was purified to an A-553/A-275 ratio of 0.8. Midpoint potential (at pH 7), isoelectric point and apparent molecular weight of the cytochrome were +0.35 V, 8.6, and around 10,500, respectively. The cytochrome proved to be an excellent electron donor to the aa3-type cytochrome oxidase in both plasma and thylakoid membranes isolated and purified from Nostoc Mac. Chemoheterotrophic growth of the cells increased the level of periplasmic cytochrome c up to 10-fold and cytochrome oxidase activity of plasma membranes up to 90-fold. The periplasmic cytochrome also transferred electrons to photosystem I in illuminated thylakoid membranes. We conclude that cyanobacteria contain a periplasmic c-type cytochrome presumably identical to so-called cytochrome c6 or c-553 which has long been known as a photosynthetic (i.e. thylakoid-associated) redox protein in these organisms, and which is capable of donating electrons (from the periplasmic space) to the cytochrome oxidase in the plasma membrane and (from the thylakoid lumen) to both P700 and cytochrome oxidase in the thylakoid membrane.