This domain is present in phytochromes and cGMP-specific phosphodiesterases. cGMP-dependent 3',5'-cyclic phosphodiesterase (<db_xref db="EC" dbkey="3.1.4.17"/>) catalyses the conversion of guanosine 3',5'-cyclic phosphate to guanosine 5'-phosphate.A phytochrome is a regulatory photoreceptor which exists in 2 forms that are reversibly interconvertible by light, the PR form that absorbs maximally in the red region of the spectrum, and the PFR form that absorbs maximally in the far-red region. This domain is also found in 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.