A plasmid cloning vehicle, pBGP120, has been constructed which permits controlled expression of cloned eucaryotic DNA in bacteria. This proposal describes experiments exploiting several novel characteristics of this plasmid in order to investigate the control of eucaryotic gene expression. Three projects are described. First, I intend to study the expression of eucaryotic ribosomal and histone genes in bacteria at both the transcriptional and translational levels. These studies are directed to the question of whether eucaryotic genes can be expressed functionally in bacteria under controlled conditions. Second, I intend to use pBGP120 to clone potentially translated regions of the Drosophila genome. The cloning technique proposed is unique in that a hitherto uncloned class of genes, viz. those represented by infrequent mRNA's, is preferentially cloned. The vast majority of expressed genes fall into this category. The acquisition of a number of such cloned sequences would facilitate an investigation of the control of their expression in Drosophila. Third, I intend to study the transcription of pBGP120 DNA-histone complexes in vitro. Such complexes, formed in vitro, are simple model chromosomes with which one can readily study the relationship between nucleosomes and ongoing transcription, the influence of histone modification on transcription, and possible interactions between nucleosomes and specific DNA sequence recognition elements, such as the lac repressor.