Specific interactions between proteins and nucleic acids are the fundamental events that effect the expression, replication and packaging of genetic elements. This proposal describes studies on the molecular aspects of such interactions. Three systems are under investigation. A. We propose to examine the mechanisms involved in the generation of spontaneous rearrangements of the orthopoxvirus genome, focusing in particular on the factors that determine the extent, the sites and the frequency of such rearrangements. We will also study the control of viral transcription by identifying and characterizing strong late promoter elements in the cowpox virus genome. We will also attempt to identify the replication origins in cowpox virus DNA. B. We will continue our studies on mutants of the 37 Kd replication initiator protein of the plasmid pSC101; we will test these mutants for their ability to replicate. We will also express in E. coli and purify the pX protein of HTLV-1 and the protein encoded by the El open reading frame of BPV. We will use for this purpose an expression vector recently constructed by us, namely the collagen vector pJG200. We will also test these proteins for sequence specific DNA-binding ability. C. We will study the mechanisms involved in the rapid and complete inhibition of the processing and assembly of small nuclear ribonucleoproteins in cells infected with vesicular stomatitis virus. This effect is associated with the accumulation of precursor forms of U1 and U2 RNA. We will determine whether splicing of messenger RNAs in general is also inhibited. We will also continue our studies on the identification and function of proteins associated with ribosomal RNA and identify the genomic organization of the genes encoding these proteins. Finally, we will sequence the La protein gene; this is the protein with which VSV leader RNA interacts in infected cells. We will map the introns and exons of this gene, as well as identify the promoters and other upstream control sequences of its mRNA.