The structure of the E. coli recA protein monomer and polymer.

The crystal structure of the recA protein from Escherichia coli at 2.3-A resolution reveals a major domain that binds ADP and probably single- and double-stranded DNA. Two smaller subdomains at the N and C termini protrude from the protein and respectively stabilize a 6(1) helical polymer of protein subunits and interpolymer bundles. This polymer structure closely resembles that of recA/DNA filaments determined by electron microscopy. Mutations in recA protein that enhance coprotease, DNA-binding and/or strand-exchange activity can be explained if the interpolymer interactions in the crystal reflect a regulatory mechanism in vivo.     

DNA loops induced by cooperative binding of lambda repressor

It has been shown by Hochschild and Ptashne that lambda repressors bind cooperatively to operator sites separated by five or six turns of the helix. Cooperative binding is not observed if the sites are separated by a nonintegral number of turns, unless a four-nucleotide gap is introduced into one of the strands between the two sites. These and other facts suggested that repressors at the separated sites touch each other, the DNA bending smoothly so as to accommodate the protein-protein interaction. Here we use electron microscopy to visualize the predicted protein-DNA complexes.     

Alternative packing arrangements in the hydrophobic core of lambda repressor

The random alteration of hydrophobic core positions in the N-terminal domain of lambda-repressor, both individually and in combination, shows that there are many ways of repacking the core of the protein. Although the number of functional sequences is limited by constraints on composition, volume and steric interactions, the simple requirement that these positions remain hydrophobic is the main determinant of whether a core sequence is compatible with the wild-type fold.     

Amino-terminal fragments of Escherichia coli lac repressor bind to DNA.

The N-terminal fragments (residues 1-51 and 1-59) obtained by selective tryptic cleavage of native lac repressor retain the ability to bind DNA. These fragments (headpieces) are monomeric and form complexes which resemble those of tetrameric repressor with non-operator DNA. But, they do not show the high specificity of repressor for operator sequences. The DNA binding has been demonstrated by filter-binding assay as well as in solution using absorption, circular dichroism, and fluorescence measurements.     

Direct role of the himA gene product in phage lambda integration

The integration of phage lambda into the Escherichia coli chromosome is accomplished by a site-specific recombination between two unique DNA sequences (attB on the bacterial genome and attP on the phage; reviewed in refs 2, 3) and requires proteins encoded by both the bacterium and the phage. Genetic and biochemical studies have shown that bacterial strains mutant in the himA gene, located at 38 min on the E. coli map, are defective in the activity of the host-encoded component. They are, moreover, defective for the growth of bacteriophage Mu, for precise excision of transposable antibiotic resistance determinants and for the synthesis of the lambda int gene product. We now show that the himA gene product (phimA) is not solely a regulator of genes involved in integration but is one of two host polypeptides required for integrative recombination.     

Structure of the cro repressor from bacteriophage lambda and its interaction with DNA

The three-dimensional structure of the 66-amino acid cro repressor protein of bacteriophage lambda suggests how it binds to its operator DNA. We propose that a dimer of cro protein is bound to the B-form of DNA with the 2-fold axis of the dimer coincident with the 2-fold axis of DNA. A pair of 2-fold-related alpha-helices of the repressor, lying within successive major grooves of the DNA, seem to be a major determinant in recognition and binding. In addition, the C-terminal residues of the protein, some of which are disordered in the absence of DNA, appear to contribute to the binding.