protein modification cytochrome c assembly PMID:2172694 pathway Bacterial c-type cytochromes are localized to the outside of the cytoplasmic membrane where they function in electron transport processes. Assembly takes place on the outside of the cytoplasmic membrane, involving separate transport of heme and the apocytochrome across the membrane [PMID:2172694]; the latter occurs via the general protein secretory (Sec) pathway of the cell [PMID:9219541]. Following translocation, assembly proceeds via a pathway involving a number of specific proteins. Two distinct systems have been identified in bacteria. System I is found in alpha - and gamma -proteobacteria, including E. coli, R. capsulatus, B. japonicum, and P. denitrificans. System II seems more widespread than system I, occurring in a range of bacteria including cyanobacteria, Gram-positive bacteria such as Mycobacterium species, beta -, gamma -, and delta -proteobacteria such as B. pertussis, T. ferrooxidans, and H. pylori, respectively, and some extremophiles such as A. aeoliticus. Common to both systems is that the apocytochrome cysteine thiols and the heme iron must be in their reduced states for thioether bond formation to occur. Thiol-disulfide oxidoreductases are required for this, and specific proteins have been identified in a number of system I organisms, including E. coli (CcmG), R. capsulatus (HelX), B. japonicum (CycY), and P. denitrificans (CcmG), and one system II organism, B. pertussis (CcsX) [PMID:12637552]. PMID:12637552 c-type cytochrome assembly PMID:9219541 UPa:UPA00555