TheInt family of site-specific recombinases: Some thoughts on a general reaction mechanism

The FLP recombinase of the yeast 2 micron circle plasmid belongs to theInt family of recombinases. Only three amino acid residues are invariant among members of this family. Functional analyses of FLP protein variants mutated at these three residues suggest their involvement at specific steps of the recombination pathway. We propose that these residues play the same functional role in the mechanism of action of all theInt family recombinases.     

Bending-incompetent variants of Flp recombinase mediate strand transfer in half-site recombinations: role of DNA bending in recombination.

One key feature of the interaction of Flp recombinase with its target site (FRT) is the large bend introduced in the substrate as a result of protein binding. The extent of bending was found to depend on the phasing and spacing of the Flp monomers occupying the two Flp-binding elements (FBE) bordering the strand-exchange region (spacer) of the substrate. The relative mobilities of the Flp complexes formed by the two permuted substrate fragments, containing the FRT site near the end or in the middle, corresponded to a DNA bend of approx. 140 degrees when each of the two FBEs flanking the spacer was occupied by a protein monomer. The estimated bend angle was the same when the reference DNA fragment with the FRT site at the end was substituted by one with the site in the middle, but containing a 4-bp insertion within the spacer. We used a combination of wild-type Flp and Flp variants that were competent or incompetent in DNA bending, together with full, or half FRT sites, to ask whether bending is a conformational requirement for catalysis, namely cleavage and exchange of strands. We obtained the following results: in full-site (FRT) vs. full-site recombinations or in full-site vs. half-site (half FRT) recombinations, there was a large difference in the reactivity between Flp and a bending-incompetent Flp variant. This difference virtually disappeared when reactions were done with half-FRT sites. We conclude that bending is not a prerequisite for catalysis, but represents the manner in which the substrate accommodates the Flp protomer-protomer interactions that are pertinent to catalysis.     

A facile enzymatic method for the location of radioactivity associated with the carbons of L-aspartic acid, L-asparagine, and beta-cyano-L-alanine.

A radiometric enzymatic technique has been devised for locating radioactivity in the carbon skeleton of l-aspartic acid, l-asparagine, and β-cyano-l-alanine. l-Asparaginase at demonstrably discriminant concentrations is used to hydrolyze l-asparagine and/or β-cyano-l-alanine to l-aspartic acid, which in turn is transaminated to oxaloacetic acid by l-glutamate oxaloacetate transaminase. The β-carboxyl group of oxaloacetate is detached by zinc ions, and the radiolabeled CO₂ is collected in alkali after diffusion. The residual pyruvic acid is α-decarboxylated by pyruvate decarboxylase to CO₂, which is collected in a second alkaline trap, and the other product of decarboxylation, acetaldehyde, diffuses into a separate semicarbazide trap. Carbons 4, 1, and 3 plus 2 are located in this order. This method is shown to be facile, sensitive, reliable, and applicable to samples of biological origin.     

Fine needle aspiration cytology of well-differentiated papillary peritoneal mesothelioma. Report of a case

The fine needle aspiration cytologic findings in a well-differentiated multifocal papillary peritoneal mesothelioma in a young man presenting with abdominal pain and mass are described. The patient is alive and well 50 months after the onset of symptomatology. The cytologic and histologic appearance as well as the clinical course of the patient point to a benign multifocal mesothelioma.     

Inhibition of L-asparagine synthetase by mucochloric and mucobromic acids.

Mucochloric and mucobromic acids are powerful inhibitors of tumoral and pancreatic L-asparagine synthetases. Two nitrogen donors, L-glutamine and ammonia, can be used by these enzymes; at a concentration of 1 mmol/l, mucochloric and mucobromic acids preferentially inhibit the utilization of ammonia as opposed to L-glutamine in vitro. Using the tumoral enzyme, kinetic analysis revealed that mucochloric acid produced inhibition which was apparently noncompetitive with ammonia but competitive with L-glutamine. In molar excess, L-glutamine and dithiothreitol effectively antagonized such inhibition; dialysis, however, failed to reverse established inhibition. These findings, suggest that the drugs operate by covalent attachment to crucial sulfhydryl functions on the enzyme.