Specific binding of Mu repressor to DNA

Abstract

PHAGE MU is a temperate virus of Escherichia coli (for reviews see refs 1, 2). The immunity system of Mu has been shown by genetic and physical mapping to lie in the furthest left, 1,000-base-pair restriction fragment of the phage DNA^3–6. This end of the phage genome is associated with a short (100-base pair) segment of bacterial DNA⁴. The HindIII-c restriction fragment of Mu (the furthest left 1,000 base pairs⁴) has been cloned into the amplifiable plasmid pMB9 (ref. 7). These recombinant plasmids confer high levels of Mu immunity on their hosts and, when recloned into minicell-producing strains, were shown to produce an additional protein of 25,000 subunit molecular weight in purified minicells^6,7. This protein is assumed to be the Mu repressor because its production is correlated with immunity to Mu. We report here experiments which show that this protein binds specifically and with high affinity to Mu DNA, as would be expected of the Mu repressor. Furthermore, we demonstrate that the DNA sequence (operator) to which the Mu repressor binds is located on the same HindIII-c restriction fragment of Mu DNA as the repressor gene.