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8.A.79 The 3 TMS cyd Operon Protein, CydX (CydX) Family 

cydX (ybgE) is a gene within the cyd operon of E. coli.  The encoded protein has 97 aas and 3 TMSs and is essential for CydAB function (VanOrsdel et al. 2013).  Homologues in other organisms have the same or a similar topology. It is requried for the assembly and stability of the diheme active site (Hoeser et al. 2014). It may play a role in a functional interaction with the CydA Q-loop, an enigmatic domain of the cytochrome bd oxidase complex (Allen et al. 2014).

References associated with 8.A.79 family:

Allen, R.J., E.P. Brenner, C.E. VanOrsdel, J.J. Hobson, D.J. Hearn, and M.R. Hemm. (2014). Conservation analysis of the CydX protein yields insights into small protein identification and evolution. BMC Genomics 15: 946. 25475368
Hoeser, J., S. Hong, G. Gehmann, R.B. Gennis, and T. Friedrich. (2014). Subunit CydX of Escherichia coli cytochrome bd ubiquinol oxidase is essential for assembly and stability of the di-heme active site. FEBS Lett. 588: 1537-1541. 24681096
VanOrsdel, C.E., S. Bhatt, R.J. Allen, E.P. Brenner, J.J. Hobson, A. Jamil, B.M. Haynes, A.M. Genson, and M.R. Hemm. (2013). The Escherichia coli CydX protein is a member of the CydAB cytochrome bd oxidase complex and is required for cytochrome bd oxidase activity. J. Bacteriol. 195: 3640-3650. 23749980