The formation of a 2-tetrahydropyridinyl-5-imidazolinone protein chromophore cross-link from the alpha-carboxyl carbon of residue n, a lysine, to the alpha-amino nitrogen of residue n+2, a glycine, and a dehydration to form a double bond to the alpha-amino nitrogen of residue n+1. This cross-linking is coupled with a dehydrogenation of residue n+1 to form a double bond between the alpha and beta carbons. In addition, the residue N lysine undergoes cyclization. The alpha-amino nitrogen is replaced by the epsilon-amino nitrogen, the peptide chain is broken, residue N-1 is released as an amide, and a double bond is formed between the alpha-carbon and the nitrogen so that a tetrahydropyridine ring results. This modification is found in the GFP-like fluorescent chromoprotein FP538 from the sea anemone Zoanthus species.