Hotspots of homologous recombination

Homologous recombination occurs at higher than average frequency at and near hotspots. Hotspots are special nucleotide sequences recognized by proteins that promote, directly or indirectly, a rate limiting step of recombination. This review focuses on two well-studied examples, the Chi sites of the bacteriumEscherichia coli and the M26 site of the fission yeastSchizosaccharomyces pombe. Chi, 5 G-C-T-G-G-T-G-G 3, is recognized by the RecBCD enzyme, which nicks the DNA near Chi and produces a 3-ended single-stranded DNA tail; this tail is a potent substrate for homologous pairing by RecA and single-stranded DNA binding proteins. M26, 5 A-T-G-A-C-G-T 3, is recognized by a heterodimeric protein and stimulates, by an as-yet-unknown mechanism, meiotic recombination at and near theade6 gene. Additional hotspots in bacteria, fungi, and mammals enhance recombination directly or indirectly via a variety of mechanisms. Although hotspots are widespread among organisms, the biological role of their localized enhancement of recombination remains a matter of speculation.     

In vitro reconstitution of homologous recombination reactions

The proteins essential to homologous recombination inE. coli have been purified and their individual activities have been identified, permitting biochemical reconstitution of steps that comprise the cellular recombination process. This review focuses on the biochemical events responsible for the initiation and homologous pairing steps of genetic recombination. The properties of an in vitro recombination reaction that requires the concerted action of recA, recBCD, and SSB proteins and that is stimulated by the recombination hotspot, Chi(), are described. The recBCD enzyme serves as the initiator of this reaction; its DNA helicase activity produces single-stranded DNA that is used by the recA protein to promote homologous pairing and DNA strand invasion of supercoiled (recipient) DNA. The SSB protein acts to trap the single-stranded DNA produced by recBCD enzyme and to facilitate pairing by the recA protein. The regulatory sequence acts incis by attenuating the nuclease, but not the helicase, activity of recBCD enzyme. This attenuation assures the preservation of ssDNA produced by the DNA helicase activity and is responsible for the simulation in vitro and, presumably, in vivo. The attenuation of nuclease activity by results in the loss or functional inactivation of the recD subunit.     

Gene conversion in the chicken immunoglobulin locus: A paradigm of homologous recombination in higher eukaryotes

Gene conversion was first defined in yeast as a type of homologous recombination in which the donor sequence does not change. In chicken B cells, gene conversion builds the antigen receptor repertoire by introducing sequence diversity into the immunoglobulin genes. Immunoglobulin gene conversion continues at high frequency in an avian leukosis virus induced chicken B cell line. This cell line can be modified by homologous integration of transfected DNA constructs offering a model system for studying gene conversion in higher eukaryotes. In search for genes which might participate in chicken immunoglobulin gene conversion, we have identified chicken counterparts of the yeastRAD51, RAD52, andRAD54 genes. Disruption and overexpression of these genes in the chicken B cell line may clarify their role in gene conversion and gene targeting.     

Cytological aspects of meiotic recombination

This article reviews current views on the mechanisms of meiotic homology searching and recombination. It discusses the relationship between molecular events at meiotic prophase and concomitant cytological processes. The role of the synaptonemal complex and other meiosis-specific structures is discussed. Whereas the relationship of crossovers, late recombination nodules, and chiasmata is well established, there is still some controversy about the temporal and causal relationships between double strand breaks, homologue recognition, heteroduplexes, early nodules and presynaptic alignment.     

Homologous recombination in plants

In plants three different approaches have been used to study homologous DNA recombination; extrachromosomal recombination (ECR) between transfected DNA molecules, intrachromosomal recombination (ICR) between repeated genes integrated into and resident at the genome and recombination between introduced DNA and homologous sequences in the genome (gene targeting). ECR is efficient (10^–1 to 10^–3) and occurs mainly during a limited time period early after transfection. It proceeds predominantly via nonconservative single-strand annealing. ICR, which in most cases is described best by the double-strand break repair model of recombination, occurs at frequencies of one event in 10⁵ to 10⁷ cells. ICR takes place throughout the whole life-cycle of a plant, in all organs and at different developmental stages. As there exists no predetermined germline in plants, somatic recombination events can be transferred to the next generation. Recombination frequencies are enhanced by DNA damage. Gene targeting, like ICR, occurs at low rates in plant cells. Almost nothing is known about the enzymes involved in homologous recombination in plants.     

Binding of cytophilic rabbit IgG to homologous hepatocytes

Summary Rabbit liver cells were able to bind cytophilic monomeric and polymeric homologous IgG via their Fc receptor binding sites (FcR). On the other hand, non-cytophilic rabbit IgG did not bind to hepatocytes, even after its aggregation. The present findings suggest that FcR on rabbit liver cells are specific for cytophilic monomeric IgG but do not significantly bind non-cytophilic, polymeric IgG.We thank Mrs Sanda Maghiar for her excellent technical assistance.     

From parasympathomimetics to parasympatholytics by homologous substitution

Zusammenfassung Auf Grund der theoretischen Befunde war zu erwarten, dass Änderungen an der negativen und positiven Seite der Dioxolan-Verbindung 2249 F zu graduellen Übergängen von Parasympathikomimetika zu Parasympathikolytika führen würden. Die Experimente bestätigten die Erwartungen vollauf.Untersuchung homologer Reihen von Alkyltrialkylammonium-Verbindungen zeigte die negative Gruppe als nicht wesentlich für die parasympathische Wirkung und ergab ebenso einen graduellen Übergang von Mimetika zu Lytika.     

Binding of cytophilic rabbit IgG to homologous hepatocytes.

Rabbit liver cells were able to bind cytophilic monomeric and polymeric homologous IgG via their Fc receptor binding sites (FcR). On the other hand, non-cytophilic rabbit IgG did not bind to hepatocytes, even after its aggregation. The present findings suggest that FcR on rabbit liver cells are specific for cytophilic monomeric IgG but do not significantly bind non-cytophilic, polymeric IgG.     

Absence of recombination in the male ofCeratitis capitata

Summary The linkage relationship between a morphological and a biochemical locus was studied. Results suggest that recombination does not occur in the male of the Mediterrannean fruit fly although its meiosis is typically chiasmatic.Publicacion técnica gen. No. 603.Acknowledgments. I would like to thank Ing. Ewald A. Favret for his critical reading of this paper and Mr and Mrs Ernesto Lifschitz for their encouragement and provision of facilities.     

Tumour immunity in rats following injection of homologous ribosomes

Riassunto Ratti trattati con ribosomi omologhi di fegato in «Freund adjuvant» sviluppano una condizione di refrattarietà al successivo trapianto di cellule di epatoma ascitico di Yoshida.     

Soluble antigen-antibody complexes reactive in homologous rabbit skin

Résumé Des complexes solubles d'antigènes-anticorps préparés à partir de l'anticorps DNP non précipitable de lapin et obtenus à des stades «avancés» et «retardés» d'immunisation produisent une réaction immédiate d'hypersensibilité cutanée quand ils sont injectés dans une peau normale de lapin. La réduction, le chauffage à 56°C et la succinilation des anticorps suggèrent que les anticorps des stades «avancés» et «retardés» compris dans les complexes diffèrent dans leurs propriétés physicochimiques, bien qu'ils possèdent le même pouvoir de sensibilisation sur les peaux homologues.     

Influence of age on calcification in homologous transplants in rats

Résumé Chez le rat, des homogreffes de peau ou d'aorte implantées dans le tissu souscutané ne se calcifient que si les receveurs sont jeunes (28 jours), quel que soit l'âge du donneur. Les dépôts calcaires sont localisés dans l'épithélium et les follicules pileux de la peau ainsi que dans les lamelles élastiques de l'aorte.