Structure-Function, Stability, and Chemical Modification of the Cyanobacterial Cytochrome b6f Complex from Nostoc sp. PCC 7120.

The crystal structure of the cyanobacterial cytochrome b(6)f complex has previously been solved to 3.0-A resolution using the thermophilic Mastigocladus laminosus whose genome has not been sequenced. Several unicellular cyanobacteria, whose genomes have been sequenced and are tractable for mutagenesis, do not yield b(6)f complex in an intact dimeric state with significant electron transport activity. The genome of Nostoc sp. PCC 7120 has been sequenced and is closer phylogenetically to M. laminosus than are unicellular cyanobacteria. The amino acid sequences of the large core subunits and four small peripheral subunits of Nostoc are 88 and 80% identical to those in the M. laminosus b(6)f complex. Purified b(6)f complex from Nostoc has a stable dimeric structure, eight subunits with masses similar to those of M. laminosus, and comparable electron transport activity. The crystal structure of the native b(6)f complex, determined to a resolution of 3.0A (PDB id: 2ZT9), is almost identical to that of M. laminosus. Two unique aspects of the Nostoc complex are: (i) a dominant conformation of heme b(p) that is rotated 180 degrees about the alpha- and gamma-meso carbon axis relative to the orientation in the M. laminosus complex and (ii) acetylation of the Rieske iron-sulfur protein (PetC) at the N terminus, a post-translational modification unprecedented in cyanobacterial membrane and electron transport proteins, and in polypeptides of cytochrome bc complexes from any source. The high spin electronic character of the unique heme c(n) is similar to that previously found in the b(6)f complex from other sources.