The long term objective of this study is to understand how oxygen regulates the expression of the tetrapyrrole biosynthetic genes. All organisms produce one or more of the tetrapyrrole derivatives, heme, chlorophyll and vitamin B12. The purple non-sulfur photosynthetic bacterium R. capsulata is one of the few organisms that synthesizes all three tetrapyrrole products. The transcription of the bacteriochlorophyll biosynthetic genes is controlled by oxygen, and there is considerable evidence to suggest that the genes of the common tetrapyrrole biosynthetic pathway are also regulated by oxygen. The specific aims of this project are (1) to isolate mutants in the tetrapyrrole biosynthetic genes; (2) isolate R'-factors carrying these genes; (3) clone the genes from the R'-factors and (4) determine which of the genes are transcriptionally controlled by oxygen. The construction of R'-factors and the subcloning of fragments from them will allow the construction of genetic and physical maps of the tetrapyrrole biosynthetic genes. Two methods will be used to determine which of the genes are regulated by oxygen. The first will involve the isolation of fusions between the Mu dl(Ap,lac) phage and the tetrapyrrole biosynthetic genes, and measurement of lacZ gene expression under high and low oxygen tensions. The second method will involve direct measurement of the levels of mRNA for the tetrapyrrole biosynthetic genes present in cells grown under various oxygen tensions. The knowledge gained from these studies will not only help in clarifying how the genes for heme, chlorophyll and vitamin B12 are regulated by oxygen, but may also help in understanding how oxygen controls other diverse systems such as nitrogen fixation and photosynthetic membrane differentiation.