Phycobilisomes are aggregates of light-harvesting proteins attached to the 
stroma side of the thylakoid membranes of the cyanobacteria (blue-green algae) 
and red algae. The water-soluble phycobiliproteins, of which there are three 
major groups, tetrapyrrole chromophores covalently bound to apoprotein. Several 
additional protiens are found within the phycobilisome and serve to link the 
phycobiliproteins to each other in an ordered fashion and also to attach the 
phycobilisome to the thylakoid membrane. Excitation energy absorbed by 
phycoerythrin is transferred through phycocyanin to allophycocyanin with an 
efficiency approximating 100%. This pathway of excitation energy transfer, 
directly confirmed by time-resolved spectroscopic measurements, has been 
incorporated into models describing the ultrastructure of the phycobilisome. The 
model for the most typical type of phycobilisome describes an 
allophycocyanin-containing core composed of three cylinders arranged so that 
their longitudinal axes are parallel and their ends form a triangle. Attached to 
this core are six rod structures which contain phycocyanin proximal to the core 
and phycoerythrin distal to the core. The axes of these rods are perpendicular 
to the longitudinal axis of the core. This arrangement ensures a very efficient 
transfer of energy. The association of phycoerythrin and phycocyanin within the 
rods and the attachment of the rods to the core and the core to the thylakoid 
require the presence of several 'linker' polypeptides. It is recently possible 
to assemble functionally and structurally intact phycobilisomes in vitro from 
separated components as well as to reassociate phycobilisomes with stripped 
thylakoids. Understanding of the biosynthesis and in vivo assembly of 
phycobilisomes will be greatly aided by the current advances in molecular 
genetics, as exemplified by recent identification of several genes encoding 
phycobilisome components.Combined ultrastructural, biochemical and biophysical 
approaches to the study of cyanobacterial and red algal cells and isolated 
phycobilisome-thylakoid fractions are leading to a clearer understanding of the 
phycobilisome-thylakoid structural interactions, energy transfer to the reaction 
centers and regulation of excitation energy distribution. However, compared to 
our current knowledge concerning the structural and functional organization of 
the isolated phycobilisome, this research area is relatively unexplored.