Bile pigments in plants and animals are structurally related by virtue of their common origin from the carbon skeleton of protoporphyrin IX, but otherwise distinct in at least two fundamental ways. First, plant bile pigments serve as photoreceptors for a host of light-mediated processes necessary for ontogenetic development of the organism whereas mammalian bile pigments are transient elimination products of hemoglobin catabolism with no known function. Second, plant bile pigments are covalently linked to specific apoproteins and the resulting chromophore-apoprotein interaction renders them insensitive to photo-destruction in vivo. Mammalian bile pigments form non-covalent (ionic, hydrophobic) bonds with proteins (e.g., albumin, ligandin) and are destroyed in high intensity light. Bile pigments in algae and mammals are synthesized from heme via a Two-Molecule Mechanism in which the lactam oxygen atoms on the external rings arise from molecular oxygen, but from different oxygen molecules. The events which occur in this process will be examined by measuring the mechanism of 18,1802 incorporation into bile pigment in red and blue-green algae. Bile pigment-apoprotein complexes (phycobiliproteins) associate into high molecular weight multimers in vivo to produce a functional aggregate called a phycobilisome. The primary structure of the allophycocyanin and phycocyanin apoproteins to which bile pigment is covalently attached will be determined by automated sequence analysis. An attempt will be made to translate the mRNA's for the apoprotein subunits of allophycocyanin and phycocyanin from a red algae to determine some of the events involved in regulation of phycobiliprotein synthesis in vivo.