It is proposed to continue the analysis of the P2 - P4 bacteriophage system consisting of P4 as satellite virus and P2 (or P2 relatives) as helper, with Escherichia coli as host. The main aim is to further elucidate the genetic control mechanisms that couple the gene expression for satellite and helper. These include mutual derepression and trans-activation. Specifically the proposal concerns: (1) further studies of cox mutations of P2 with regard to their effects on the derepression of P2 and P4; (2) further studies of mutation rpoA109 (equals gro109) in the gene for the alpha subunit of RNa polymerase with regard to its effects on gene expression by P2 and P4; (3) studies of a recently isolated bacterial mutant with a mutation (groPC259) tentatively assigned to gene dnaJ, with regard to the effects of groPC259 on P2 and P4; (4) a search for P2 mutants deficient in reciprocal transactivation of P4; (5) studies of P4's interference with the growth of P2; (6) attempts to detect gene activation by P4 for phage lambda genes and for a bacterial operon (P4 suppresses polar effects of amber mutations in the P2 genome, presumably by allowing transcription to override termination signals, hence P4 may also exert an "antitermination effect" with regard to transcription of genes not belonging to a helper genome); (7) a search for P2 - P3 DNA tandems formed in vivo (if they exist they could be of crucial importance for P2- P4 interactions); (8) further P4-related studies: polarity suppression by the transactivation defective mutant P4 6, characterization of recently isolated bacterial mutants that allow P2 to grow but not P4, search for P4 mutants.