A probabilistic model for genetic recombination of nonreplicating lambda-phage DNA, stimulated by "mismatch repair" of UV photoproducts.

Genetic recombination of nonreplicating phage lambda-DNA, during infection of homoimmune lysogenic bacteria, was previously observed to be dramatically stimulated by prior uv irradiation of the phages, even when the Escherichia coli hosts lacked the major uv-photo-product excision-repair system (UvrABC). UvrABC-independent recombination of circular phage molecules depends on host MutHLS functions and on undermethylation of adenines at GATC sites in the phage DNA, and thus appears to be the result of "mismatch repair" of uv photoproducts. Recombinant frequencies pass through a relatively sharp maximum at 20 J/m2 and decrease at higher doses, whereas most plausible models for the process predict monotonic increases with dose, or a plateau at high uv doses. A uv-dose-dependent loss of biological activity (restriction) of all intracellular phage DNA was also observed previously. In order to provide a framework for testing possible explanations for the unusual recombinant-frequency vs uv-dose curve, a statistical model was constructed. This model includes probability terms for all possible one-exchange and two-exchange recombination processes, and incorporates the assumption that dimer recombinants are more susceptible to restriction than monomer parents (or recombinants), because of their larger target size. By adjustment of model parameters, particularly epsilon, the efficiency per photoproduct of initiation of a recombinational exchange, a theoretical dose-response curve that agreed well with experiment was obtained. The best fit corresponded to epsilon = 0.035, close to the previously observed restriction efficiency of 0.053. In the calculations, the value for h0, the average length of heteroduplex DNA, was taken to be 0.5 lambda units, i.e., about 25 kilobase pairs. This estimate for h0 was obtained here by analysis of the density distributions of the progeny of crosses between nonreplicating density-labeled lambda-phage chromosomes, published by others [M. S. Fox, C. S. Dudney and E. J. Sodergren (1979) Cold Spring Harbor Symposium on Quantitative Biology, Vo. 43, pp. 999-1007].     

PHH1, a novel gene from Arabidopsis thaliana that encodes a protein similar to plant blue-light photoreceptors and microbial photolyases

A cDNA from Arabidopsis thaliana similar to microbial photolyase genes, and designated AT-PHH1, was isolated using a photolyase-like cDNA from Sinapsis alba (SA-PHR1) as a probe. Multiple isolations yielded only PHH1 cDNAs, and a few blue-light-receptor CRY1 (HY4) cDNAs (also similar to microbial photolyase genes), suggesting the absence of any other highly similar Arabidopsis genes. The AT-PHH1 and SA-PHR1 cDNA sequences predict 89% identity at the protein level, except for an AT-PHH1 C-terminal extension (111 amino acids), also not seen in microbial photolyases. AT-PHH1 and CRY1 show less similarity (54% protein identity), including respective C-terminal extensions that are themselves mostly dissimilar. Analysis of fifteen AT-PHH1 genomic isolates reveals a single gene, with three introns in the coding sequence and one in the 5′-untranslated leader. Full-length AT-PHH1, and both AT-PHH1 and AT-PHH1ΔC-513 (truncated to be approximately the size of microbial photolyase genes) cDNAs, were overexpressed, respectively, in yeast and Escherichia coli mutants hypersensitive to ultraviolet light. The absence of significant effects on resistance suggests either that any putative AT-PHH1 DNA repair activity requires cofactors/chromophores not present in yeast or E. coli, or that AT-PHH1 encodes a blue-light/ultraviolet-A receptor rather than a DNA repair protein.     

The Ethics of Medical Practitioner Migration From Low-Resourced Countries to the Developed World: A Call for Action by Health Systems and Individual Doctors

Medical migration appears to be an increasing global phenomenon, with complex contributing factors. Although it is acknowledged that such movements are inevitable, given the current globalized economy, the movement of health professionals from their country of training raises questions about equity of access and quality of care. Concerns arise if migration occurs from low- and middle-income countries (LMICs) to high-income countries (HICs). The actions of HICs receiving medical practitioners from LMICs are examined through the global justice theories of John Rawls and Immanuel Kant. These theories were initially proposed by Pogge (1988) and Tan (1997) and, in this work, are extended to the issue of medical migration. Global justice theories propose that instead of looking at health needs and workforce issues within their national boundaries, HICs should be guided by principles of justice relevant to the needs of health systems on a global scale. Issues of individual justice are also considered within the framework of rights and social responsibilities of individual medical practitioners. Local and international policy changes are suggested based on both global justice theories and the ideals of individual justice.     

Formation of colonies in soft agar medium by regenerating liver cells (38591).

The culture of actively regenerating liver tissue in soft agar reveals the presence of a cell population, not previously described, which includes (a) cells capable of forming hematopoietic colonies and (b) additional cells producing colonies with epithelioid characteristics.     

Persistence length of DNA

In the wormlike chain (Kratky-Porod) model of DNA the stiffness of the chain is determined by the persistence length, a. The persistence length may be evaluated from light-scattering measurements of the molecular weight and the mean-square radius if the samples are not polydisperse or if the polydispersity can be quantitatively determined. The persistence length can also be evaluated with the aid of hydro dynamic theory from measurements of intrinsic viscosity and sedimentation coefficient. Data taken from the literature and from other studies by the authors are examined by these methods. The light-scattering method yields a value of a of 900 ± 200 Å the hydrodynamic data yield 600 ± 100 Å. These values are considerably larger than those obtained by most previous authors.     

Synchronization of parturition in beef cattle with prostaglandin and dexamethasone

The effectiveness of dexamethasone and prostaglandin in combination for induction and synchronization of parturition in cattle was evaluated in 100 pregnant Angus, Hereford, Charolais and Simmental cows. Cows were distributed equally by breed, day of gestation and cow age to one of three treatments: 1) Control, 2) Dexamethasone (25 mg) plus prostaglandin F(2alpha) (25 mg) or 3) Dexamethasone (25 mg) plus fenprostalene (1 mg). Hormones were administered simultaneously from 275 to 283 d of gestation. Gestation length at calving for control cows differed significantly (P < 0.01) among breeds: Angus, 278.5 +/- 0.9; Hereford, 283.1 +/- 1.1; Charolais, 283.2 +/- 1.5; and Simmental, 285.4 +/- 1.2 d. For hormone-treated cows, 80% of the calves were born between 30 and 46 h after the hormone injections; overall mean was 37.6 +/- 1.1 h. Calving response did not differ (P >0.1) between cows treated with prostaglandin F(2alpha) versus fenprostalene (36.5 +/- 1.6 vs 38.6 +/- 1.6 h) or among cow age, day of gestation, or breed. Also, duration of labor, calving difficulty and calf viability did not differ between calves born at an induced or spontaneous parturition. The incidence of placenta retained for >24 h was higher for induced than spontaneous parturition (21.0 vs 0.0%), but it did not differ (P >0.1) between cows treated with prostaglandin F(2alpha) or fenprostalene (19.2 vs 22.6%). An acceptable degree of synchrony of parturition was attained by the administration of prostaglandin F(2alpha) or fenprostalene in combination with dexamethasone. The higher incidence of retained placenta in treated than control cows did not affect subsequent fertility. The longer biological half-life for fenprostalene than for prostaglandin F(2alpha) provided no improvement in increasing synchrony of parturition or decreasing frequency of retained placenta.