Binds to and releases the BluR repressor from its bound DNA target in a blue light-dependent (470 nm) fashion. A shift to low temperature also triggers a BluF-mediated relief of repression by BluR, suggesting BluF may serve as a thermometer. Blue light may act to increase the affinity of BluF for BluR, allowing it to be released from its operator. The protein has a reversible photocycle, and undergoes structural changes, probably in the EAL domain, in response to light. Binds 1 FAD per subunit, non-covalently bound to the BLUF domain. Monomer, it undergoes transient dimerization following photoexcitation or upon temperature reduction, with a relaxation time of about 2 minutes. The dimer may be the inactive state. Interacts with the N- and C-terminal domains of BluR. Can also interact with the C-terminal domain of MlrA. Expression is strongly activated by cold shock (over 30-fold at 16 degrees Celsius compared to 37 degrees) at low temperature in a PNPase-dependent fashion. Repressed by RpoS. The joining helix is required for stability of the light-adapted state. Contains a defective C-terminal cyclic-di-GMP phosphodiesterase EAL domain, which lacks key amino acids required for phosphodiesterase activity. Restoration of consensus amino acids in the degenerate EAL domain does not restore phosphodiesterase activity and reduces ability to antagonize BluR. Reduces expression of ycgZ in the presence but not the absence of BluR. At 16 degrees Celsius, reduces expression of genes (including ycgZ) for several small proteins. While the BluR/F system is induced at low temperatures under blue light irradiation, it is not essential for growth under these conditions.