We propose to initiate studies to define mechanisms controlling gene expression in cyanobacteria. Specifically, we propose to isolate and characterize cloned fragments of cyanobacterial DNA encoding for the apoproteins of Synechococcus 7002 allophycocyanin and Synechocystis 6701 phycoerythrin. These cloned DNA fragments, together with cloned fragments already being characterized which carry coding sequences for the apoprotein subunits of Synechcoccus 7002 phycocyanin, will be used as probes to identify and isolate the mRNAs coding for these phycobiliproteins. We will directly assess the role of transcriptional control in the regulation of phycobiliprotein biosynthesis by measuring the intracellular concentrations of biliprotein mRNAs in cells exposed to various environmental stresses including nutrient starvation (for nitrogen, carbon, or phosphate), light intensity (intensity adaptation), and light wavelength (complementary chromatic adaption). We will use cloned cyanobacterial DNA fragments as templates for the development of an in vitro transcription/translation system. Such a system could be useful in the isolation of factors involved in the control of expression of the phycobiliprotein genes. The proposed studies are relevant and of comparative interest in understanding gene regulation processes in both procaryotic and eucaryotic photoautotrophs.