Ammonia-oxidizing bacteria are rich in additional cytochromes (Hooper et al. 1997). Cytochrome C554 is a unique periplasmic tetraheme cytochrome that can accept electrons from HAO (Figure 2). There are three copies of the genes that code for this cytochrome (hey or eye) located about 1200 bp downstream of each copy of hao and are transcribed separately from hao (Bergmann el al. 1994; Hommes et al. 1994; Sayavedra-Soto et al. 1996). Two copies have been sequenced and were found to be identical (Bergmann et al. 1994; Hommes et al. 1994). A putative transcription start site was determined for hey 97 bp upstream of the start codon and was associated with a ct70 promoter sequence (Hommes et al. 2001). Another membrane-bound cytochrome is encoded by genes contiguous with two of the three copies of eye (Bergmann et al. 1994) (Figure 2). Cytochrome Cm552 contains one heme and is also likely to be involved in electron transfer from HAO, albeit mediated through cytochrome C554 (Hooper et al. 1997). A soluable cytochrome C552 involved with ammonia oxidation has also been identified. The amino acid sequence and solution structure of this C552 have been determined (Fujiwara et al. 1995; Timkovich et al. 1998).
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