2] Fnr System (Fumarate nitrate reduction) :

2] Fnr system (Fumarate nitrate reduction) :

Fnr system stimulates the transcription of several "anaerobic" genes and represses the transcription of some "aerobic" genes.

When E.coli is shifted from aerobic to anaerobic growth, a number of genes are induced while others are repressed.

                          Fnr System

Fnr genes codes for protein (Fnr) that acts as that are expressed only during anerobic growth and repressor for certain genes that are expressed only during aerobic growth (cytochrome d oxidase, cytochrome O oxidase).

Genes whose expression requires for Fnr includes anaerobic respiratory enzymes fumarate reductase & nitrate reductase as well as several other enzymes including formate dehydrogenase.

Mutations in the Fnr gene result in an inability to grow anaerobically on fumarate or nitrate as electron acceptors.

Fnr is not phosphorylated or part  of two-component regulatory system.

Fnr protein is present in both aerobically and anaerobically  growing cells and it is inactive state during aerobic growth.

Fnr become activated by reduction of bound ferric ion causing confirmational change in the protein.

Possible targets for reductant are sulf-hydral  groups & iron atoms that are part of purified protein.

Parallel exists between Fnr protein & another transcriptional regulators protein, cAMP receptor protein Crp.

Crp protein is positive transpcriptional for catabolite-sensitive genes (i.e. those genes repressed by glucose).

Crp in response to binding to cAMP, bind to specific sites on the promotor region of target gene to activate transcription.