Activation of bioluminescence by structural complementation

A peptide comprising an amino acid sequence having greater than 70% sequence identity with SEQ ID NO:

• 390 or SEQ ID NO:
• 2271, but less than 100% sequence identify with SEQ ID NO:
• 2, wherein a bioluminescent signal produced in the presence of a furimazine substrate is increased when the peptide contacts a polypeptide consisting of SEQ ID NO:
• 440 when compared to a bioluminescent signal produced by the peptide and furimazine substrate alone, and wherein the amino acid sequence of the peptide is not a naturally occurring protein or a fragment thereof.