This domain is present in cGMP-specific phosphodiesterases, adenylyl and guanylyl cyclases, phytochromes, FhlA and NifA. Adenylyl and guanylyl cyclases catalyse ATP and GTP to the second messengers cAMP and cGMP, respectively, these products up-regulating catalytic activity by binding to the regulatory GAF domain(s). The opposite hydrolysis reaction is catalysed by phosphodiesterase. cGMP-dependent 3',5'-cyclic phosphodiesterase catalyses the conversion of guanosine 3',5'-cyclic phosphate to guanosine 5'-phosphate. Here too, cGMP regulates catalytic activity by GAF-domain binding. Phytochromes are regulatory photoreceptors in plants and bacteria which exist in two thermally-stable states that are reversibly inter-convertible by light: the Pr state absorbs maximally in the red region of the spectrum, while the Pfr state absorbs maximally in the far-red region. This domain is also found in FhlA (formate hydrogen lyase transcriptional activator) and NifA, a transcriptional activator which is required for activation of most Nif operons which are directly involved in nitrogen fixation. NifA interacts with sigma-54. This domain can bind biliverdine and phycocyanobilin (Matilla et al., FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043).

1: The GAF domain: an evolutionary link between diverse phototransducing proteins. Aravind L, Ponting CP; Trends Biochem Sci 1997;22:458-459. PMID:9433123

2: Cyclic nucleotide binding GAF domains from phosphodiesterases: structural and mechanistic insights. Heikaus CC, Pandit J, Klevit RE; Structure. 2009;17:1551-1557. PMID:20004158

3: Role of integration host factor in stimulating transcription from the sigma 54-dependent nifH promoter. Santero E, Hoover TR, North AK, Berger DK, Porter SC, Kustu S; J Mol Biol. 1992;227:602-620. PMID:1404379

4: Phytochrome three-dimensional structures and functions. Hughes J; Biochem Soc Trans. 2010;38:710-716. PMID:20298248