Split photoactive yellow protein complementation system and uses thereof

A complementation system comprising a first photoactive yellow protein (PYP) fragment and a second photoactive yellow protein (PYP) fragment, wherein:

• the first PYP fragment comprises an amino acid sequence of SEQ ID NO:
• 23, or an amino acid sequence having at least 70% identity with SEQ ID NO:
• 23, wherein said first PYP fragment has substitutions Y94W, T95M, F96I, D97P, Y98T, Q99S, M100R, and T101G with reference to SEQ ID NO:
• 22;
• and
• the second PYP fragment comprises an amino acid sequence selected from the group consisting of SEQ ID NO:
• 34, SEQ ID NO:
• 35, SEQ ID NO:
• 36, SEQ ID NO:
• 37, SEQ ID NO:
• 38, SEQ ID NO:
• 39, and SEQ ID NO:
• 40, or a truncated fragment thereof, wherein said truncated fragment comprises 9 or 10 consecutive amino acids from the N-terminal end of the amino acid sequence of any one of SEQ ID NOs:
• 34-40;
• wherein the first PYP fragment and the second PYP fragment or a functional fragment thereof reconstitute a functional PYP that can bind a fluorogenic hydroxybenzylidene rhodanine (HBR) analog.