Bicarbonate biosensors, calcium biosensors, and uses thereof

A biosensor for a ligand, comprising an Anabaena variabilis ligand-binding protein (avBicarbBP5) and a reporter group that transduces a detectable signal, wherein the reporter group is attached to the ligand-binding protein so that a signal transduced by the reporter group when the ligand-binding protein is bound to ligand differs from a signal transduced by the reporter group when the ligand-binding protein is not bound to ligand, wherein the ligand comprises a calcium (CaII)-bicarbonate complex (CaII—HCO3) or free calcium ion (Ca2+), and wherein the ligand-binding protein does not comprise an enzyme, wherein the ligand-binding protein comprises a mutation comprising one or more of the following substitutions:

• I16X, P17X, I18X, W49X, Q71X, C96X, F140X, T141X, F142X, P143X, N146X, T190X, and W194X, wherein X is an amino acid that results in a conservative substitution or a cysteine, of the protein avBicarbBP5 (SEQ ID NO:
• 5), and where each position is counted in avBicarbBP5 with the signal peptide replaced with a methionine (SEQ ID NO:
• 79).