The present invention relates to a nucleic acid molecule encoding a polypeptide having a fluorescence emission activity with a maximum emission at 505 to 515 nm, wherein said nucleic acid molecule is selected from the group consisting of (a) a nucleic acid molecule encoding a polypeptide having the amino acid sequence of SEQ ID NO: 2; (b) a nucleic acid molecule having the DNA sequence of SEQ ID NO: 1; (c) a nucleic acid molecule hybridizing under stringent conditions to the complementary strand of (i.) a nucleic acid molecule of (a), wherein said nucleic acid molecule of (c) encodes a polypeptide having at the position corresponding to position 146 of SEQ NO:2 a phenylalanine and at the position corresponding to position 203 of SEQ NO:2 a threonine; or (ii) a nucleic acid molecule of (b), wherein said nucleic acid molecule of (c) has at the positions corresponding to positions 438 to 440 of SEQ ID NO: 1 a nucleotide triplet selected from the group consisting of TTT and TTC; and at the positions corresponding to positions 609 to 611 of SEQ ID NO: 1 a nucleotide triplet selected from the group consisting of ACT, ACC, ACA, ACG; wherein the polypeptide encoded by the nucleic acid molecule of (c) has a fluorescence enhanced by at least the factor of 2.5 as compared to the polypeptide having the amino acid sequence of SEQ ID NO: 10; or (d) a nucleic acid molecule degenerate with respect to the nucleic acid molecule of (b). The present invention furthermore relates to a polypeptide encoded by the nucleic acid molecule of the invention, a vector and a host cell comprising the nucleic acid molecule of the invention, a method of producing said polypeptide, a fusion protein comprising the polypeptide of the invention and methods of detecting the presence and/or localization of a protein of interest and methods of detecting the activity of a promoter.