ABSTRACT
This chapter presents new functional information about cytochrome (Cyt) c 6 and its isoforms found in cyanobacteria, as well as recent data about the related higher-plant Cyt c 6A and cyanobacterial Cyt c M. Cyt c 6 is the protein responsible for the electron transfer from Cyt b 6 f complex to photosystem I (PSI) in the thylakoid lumen of cyanobacteria and some green algae, replacing plastocyanin (Pc) under copper deficiency conditions (De la Rosa et al. 2002; Hervas et al. 2003). This protein has been thought to be involved in photosynthesis (Kerfeld et al. 1999; Kerfeld and Krogmann 1998), respiration (Peschek 1999), and anoxygenic photosynthesis, in which Cyt c 6 could transport electrons between quinones and some sulfoferric clusters during anaerobic oxidation of sulfur (Garlick et al. 1977; Padan 1979). However, it remains unclear whether these processes involve one or several isoforms (Ki 2005). New systems for genome sequencing have uncovered three genes that could encode up to three possible isoforms of Cyt c 6 in some cyanobacteria, such as in the case of Anabaena variabilis or Nostoc sp. PCC 7119. The presence of isogenes opens the door to the possibility that each one could perform a different metabolic function within a cyanobacterium. The first to be found, the petJ gene, encodes for the native Cyt c 6, widely studied in several organisms and present in all cya-nobacterial genomes sequenced. The second one encodes a Cyt c 6-like protein (herein after Cyt c 6-2), which cannot oxidize Cyt b 6 f complex but can reduce, with low efficiency, PSI (Reyes-Sosa et al. 2011), and the third one encodes for Cyt c 6-3, found only in heterocyst-forming filamentous cyanobacteria (Torrado et al. 2015).
