Biochemistry of homologous recombination in Escherichia coli.

Homologous recombination is a fundamental biological process. Biochemical understanding of this process is most advanced for Escherichia coli. At least 25 gene products are involved in promoting genetic exchange. At present, this includes the RecA, RecBCD (exonuclease V), RecE (exonuclease VIII), RecF, RecG, RecJ, RecN, RecOR, RecQ, RecT, RuvAB, RuvC, SbcCD, and SSB proteins, as well as DNA polymerase I, DNA gyrase, DNA topoisomerase I, DNA ligase, and DNA helicases. The activities displayed by these enzymes include homologous DNA pairing and strand exchange, helicase, branch migration, Holliday junction binding and cleavage, nuclease, ATPase, topoisomerase, DNA binding, ATP binding, polymerase, and ligase, and, collectively, they define biochemical events that are essential for efficient recombination. In addition to these needed proteins, a cis-acting recombination hot spot known as Chi (chi: 5'-GCTGGTGG-3') plays a crucial regulatory function. The biochemical steps that comprise homologous recombination can be formally divided into four parts: (i) processing of DNA molecules into suitable recombination substrates, (ii) homologous pairing of the DNA partners and the exchange of DNA strands, (iii) extension of the nascent DNA heteroduplex; and (iv) resolution of the resulting crossover structure. This review focuses on the biochemical mechanisms underlying these steps, with particular emphases on the activities of the proteins involved and on the integration of these activities into likely biochemical pathways for recombination.     

Crystallographic structure of a multiple β-turn containing,glycine-rich heptapeptide: A synthetic precursor of the lipopeptaibol antibiotic Trichodecenin I

In continuation of our studies on the structure and function of peptaibol antibiotics, the conformational properties of a sequence analogous to that of Trichodecenin I (Z-Gly-Gly-D -Leu-Aib-Gly-D -Ile-D -Leu-OMe, where Z = benzyloxycarbonyl, Aib = α-aminoisobutyric acid, and OMe = methyl ester) have been investigated crystallographically. This sequence is the mirror image of the naturally occurring molecule and also of the C-terminal heptapeptide of the related lipo-peptaibol Trichogin A IV (where, however, the Leu-OMe residue has replaced the original Leuol residue). The molecule crystallized in the monoclinic system, space group P2₁, Z = 4, and cell parameters a = 11.610(5), b = 33.342(8), c = 11.735(4) Å, β = 110.42(1)*, V = 4257(3) ų. The crystallographic refinement converges at residual values of R = 0.047 and wR₂ = 0.134 on F². In the 1–5 segment the molecular conformation is virtually identical to that one reported from solution nmr studies of a similarly protected sequence [Biopolymers (1995), Vol. 35, pp. 21–29)] and is characterized by β-turns of type I at Gly1-Gly2, II′ at Leu3-Aib4, and I at Aib4-Gly5. In the crystal structure, a β-sheet-like arrangement is seen at the C-terminus. © 1996 John Wiley & Sons, Inc.     

Transgenic mosquitoes: a future vector control strategy?

Transgenic mosquitoes may provide a new way of dealing with the old problem of diseases transmitted by insects. Although many technical, and perhaps ethical, problems associated with the wild-release of transgenic insects have yet to be overcome, Julian Crampton and colleagues explore the potential of this technology in the continuing battle to control insect-borne disease.     

Progesterone induces expression of endometrial messenger RNA encoding for cyclooxygenase (sheep)

Exogenous progesterone given early in the ovine estrous cycle results in precocious luteolysis. It has been suggested that under this circumstance regression of the corpus luteum is caused by an advancement in the timing of uterine secretion of prostaglandin F2 alpha. Uterine tissues were obtained from ewes following administration of progesterone (or injection vehicle), fixed in paraformaldehyde, and embedded in paraffin. Tissue sections were hybridized using an 35S-labeled cRNA probe specific for cyclooxygenase mRNA. There was approximately an eight-fold increase in the level of hybridization signal, localized mainly to uterine glands, consequential to treatment with progesterone. Thus, progesterone appears to control expression of the endometrial gene and(or) the stability of the message encoding for a rate-limiting enzyme involved in metabolism of arachidonic acid to its luteolytic product, thereby resolving the length of the nonpregnant cycle.     

Paramutation in maize

Paramutation is a heritable change in gene expression induced by allele interactions. This review summarizes key experiments on three maize loci, which undergo paramutation. Similarities and differences between the phenomenology at the three loci are described. In spite of many differences with respect to the stability of the reduced expression states at each locus or whether paramutation correlates with DNA methylation and repeated sequences within the loci, recent experiments are consistent with a common mechanism underlying paramutation at all three loci. Most strikingly, trans-acting mutants have been isolated that prevent paramutation at all three loci and lead to the activation of silenced Mutator transposable elements. Models consistent with the hypothesis that paramutation involves heritable changes in chromatin structure are presented. Several potential roles for paramutation are discussed. These include localizing recombination to low-copy sequences within the genome, establishing and maintaining chromatin domain boundaries, and providing a mechanism for plants to transmit an environmentally influenced expression state to progeny.     

Evaluating the costs of mosquito resistance to malaria parasites

Costly resistance mechanisms have been cited as an explanation for the widespread occurrence of parasitic infections, yet few studies have examined these costs in detail. A malaria-mosquito model has been used to test this concept by making a comparison of the fitness of highly susceptible lines of mosquitoes with lines that are resistant to infection. Malaria infection is known to cause a decrease in fecundity and fertility of mosquitoes; resistant mosquitoes were thus predicted to be fitter than susceptible ones. Anopheles gambiae were selected for refractoriness/resistance or for increased susceptibility to infection by Plasmodium yoelii nigeriensis. Additional lines that acted as controls for inbreeding depression were raised in parallel but not exposed to selection pressure. Selections were made in triplicate so that founder effects could be detected. Resistance mechanisms that were selected included melanotic encapsulation of parasites within 24 h postinfection and the complete disappearance of parasites from the gut. Costs of immune surveillance were assessed after an uninfected feed, and costs of immune deployment were assessed after exposure to infection and to infection and additional stresses. Mosquito survivorship was unaffected by either resistance to infection or by an increased burden of infection when compared with low levels of infection. In most cases reproductive fitness was equally affected by refractoriness or by infection. Resistant mosquitoes did not gain a fitness advantage by eliminating the parasites. Costs were consistently associated with larval production and egg hatch rate but rarely attributed to changes in blood feeding and never to changes in mosquito size. No advantages appeared to be gained by the offspring of resistant mosquitoes. Furthermore, we were unable to select for refractoriness in groups of mosquitoes in which 100% or 50% of the population were exposed to infection every generation for 22 generations. Under these selection pressures, no population had become completely refractory and only one became more resistant. Variations in fitness relative to control lines in different groups were attributed to founder effects. Our conclusion from these findings is that refractoriness to malaria is as costly as tolerance of infection.