In cyanobacteria, light energy is mainly harvested by the phycobiliproteins that form the phycobilisome rods, and funneled to the photosynthetic reaction centers through the core components. Among them, allophycocyanin (alpha AP, beta AP) and the so-called LCM play a major role. This report deals with the characterization of the apcE gene from Synechococcus sp. PCC 6301 which specifies the LCM. It maps upstream from the apcA gene (alpha AP). Transcriptional analyses demonstrate that the apcABC gene cluster (alpha AP, beta AP, and LC7.8) forms an operon, whereas the apcE gene behaves as a monocistronic unit. The functional organization of the apcEABC gene cluster, as well as of the apcE gene product, of Synechococcus 6301 are compared to their counterparts in three other organisms. Finally, a model is proposed for the architecture of the phycobilisome core.