Progress has been made in studies of (1) chaperone-mediated activation of a DNA-binding protein, (2) dissociation of DNA-protein complexes, (3) DNA silencing, (4) a case of programmed cell death in a bacterial population and (5) an unusual arrangement of lamdoid phage regulatory elements in an unusual organism. The biological materials have remained plasmid prophages of E. coli, one annular (P1), the other linear with covalently sealed ends (N15). (1) New support has been obtained for the claim that chaperones may be required for refolding inactive monomeric P1 replication initiator protein (RepA) to generate the active form. Monomerization alone appears insufficient. The support comes from measurements of sedimentation velocity of dilute RepA solutions, from kinetic measurements of RepA by dynamic light scattering following RepA dilution and from the properties of dimerization-defective and chaperone-independent mutant proteins. (2) A striking dependence of DNA-RepA complex dissociation on the availability of acceptor DNA has been demonstrated, a dependence that promises to have a significant role in plasmid copy number control. (3) Mutant forms of ParB altered in their interaction with parS (the P1 centromere) have been identified and a subset selected that appear likely to help in determining the nature of the switch that can occur between plasmid partition function and dysfunction. (4) An autoregulatory circuit has been partially characterized in the P1 toxin-antitoxin system that programs the death of E. coli from which P1 has been lost. (5) A major repressor protein of N15 and its binding sites have been defined and its mode of action elucidated.
