Mechanism of photeractivation of pseudorabies virus

The social health impact of Eritrean refugees on the host communities: the case of May-ayni refugee camp, Northern Ethiopia

Objective

Migration is a contemporary global issue and the exodus of refugees may potentially posit new social health challenges in the host communities. Ethiopia is a host to the second largest refugee population in Africa. The aim of this study was to explore the health impacts by the Eritrean refugees in May-ayni camp, North West Tigrai on the host community. The research used a qualitative exploratory approach. Participants were recruited using a purposive sampling technique. The primary sources of the data were in-depth interviews of 20 key informants, and focus group discussions with 30 refugees and 30 members of the host community. Transcription and translation was done verbatim and finally thematic analysis was done using an inductive approach.

Results

The findings of this research indicated that the refugees in the May-ayni camp created actual social and health threats to the members of the host communities. The socio-cultural norms of the host peoples were disrupted in terms of their social insecurity and introduction of health related challenges such as the spread of sexually transmitted infections and other reproductive health problems.

Photoreactivation and ultraviolet-enhanced reactivation of ultraviolet-irradiated nuclear polyhedrosis virus by insect cells

The nuclear polyhedrosis virus (Baculovirus) of Galleria mellonella (Pryalidae: Lepidoptera) was used to investigate the capability of cultured insect cells to repair ultraviolet (UV) induced damage in the viral genome. When assayed by the formation of plaques in the cell line TN-368, the survival of the virus was found to decrease linearly with increased ultraviolet exposure. The infectious capacity of UV-irradiated virions was significantly restored after exposing the TN-368 monolayers to either photoreactivation conditions (white fluorescent and black light) or to UV-enhanced reactivation conditions (far ultraviolet radiation). Using both types of repair sequentially resulted in higher reactivation than when either was used alone. These results indicate that pyrimidine dimers are the major factor responsible for inactivation of this virus by UV radiation but that other photolesions not repairable by photoreactivation partially account for the inactivation of the virus.

Stimulation of host DNA synthesis and induction of early antigens by ultraviolet light irradiated human cytomegalovirus

The ultraviolet (UV-)sensitivity of the human cytomegalovirus (HCMV) genes coding for very early complement fixing and early antigens in human embryonic fibroblasts (HEF) and mouse embryonic fibroblasts (MEF) and the relation of these genes to the ability of the virus to stimulate host cell DNA synthesis were investigated. After 14 minutes of UV-irradiation of the virus inoculum only the very early complement fixing nuclear antigen (CMNA) developed in the HEF cells and only the early cytoplasmic antigen(s) was present in the MEF. In both HEF and MEF, host cell DNA synthesis was stimulated. We conclude that the ability of HCMV to stimulate host DNA synthesis is an early function of the viral genome and shows a high resistance to UV-irradiation. There is no direct correlation, however, between the ability of the virus to stimulate host cell DNA synthesis and the genes which code for the CMNA or for early cytoplasmic antigens.

Integration of proviral DNA in chicken cells infected with Schmidt-Ruppin Rous sarcoma virus is not enhanced by DNA repair

The effect DNA repair might have on the integration of exogenous proviral DNA into host cell DNA was investigated by comparing the efficiency of proviral DNA integration in normal chicken embryonic fibroblasts and in chicken embryonic fibroblasts treated with UV or 4-nitroquinoline-1-oxide. The cells were treated with UV or 4-nitroquinoline-1-oxide at various time intervals ranging from 6 h before to 24 h after infection with Schmidt-Ruppin strain A of Rous sarcoma virus. The chicken embryonic fibroblasts were subsequently cultured for 18 to 21 days to ensure maximal integration and elimination of nonintegrated exogenous proviral DNA before DNA was extracted. Integration of proviral DNA into the cellular genome was quantitated by hybridization of denatured cellular DNA on filters with an excess of (3)H-labeled 35S viral RNA. The copy number of the integrated proviruses in normal cells and in infected cells was also determined from the kinetics of liquid RNA-DNA hybridization in DNA excess. Both RNA excess and DNA excess methods of hybridization indicate that two to three copies of the endogenous provirus appear to be present per haploid normal chicken cell genome and that two to three copies of the provirus of Schmidt-Ruppin strain A of Rous sarcoma virus become integrated per haploid cell genome after infection. The copy number of viral genome equivalents integrated per cell treated with UV or 4-nitroquinoline-1-oxide at different time intervals before or after infection did not differ from the copy number in untreated but infected cells. This finding supports our previous report that the integration of oncornavirus proviral DNA is restricted to specific sites in the host cell DNA and suggests a specific mechanism for integration.

Postreplication repair of DNA in chick cells: studies using photoreactivation

During replication of DNA after ultraviolet irradiation, gaps are left in the newly-synthesized DNA strands in both bacterial and animal cells and these gaps are subsequently sealed by a process known as postreplication repair. In order to test whether it is the ultraviolet-induced pyrimidine dimers which are responsible for the production of these daughter-strand gaps in animal cells, we have used chick embryo fibroblasts. In these cells the pyrimidine dimers are photoreactivable, i.e. they can be split by an enzymatic process dependent on visible or near ultraviolet light. Our results indicate that chick cells possess a postreplication repair system similar to that in mammalian cells; gaps are produced in the newly-synthesized strands and then filled in. If the ultraviolet-irradiated cells are first photoreactivated to remove most of the dimers, the number of daughter-strand gaps produced is much less than without photoreactivation. This suggests that the dimers are indeed responsible for the formation of many of the gaps in the newly-synthesized DNA. Ultraviolet light also inhibits the overall rate of DNA synthesis. This inhibition is, however, only partly overcome by photoreactivation.

The human leukocyte photoreactivating enzyme

A photoreactivating enzyme from human leukocytes has been isolated and characterized. The enzyme requires DNA irradiated with ultraviolet light (220-300 nm) as substrate, and visible light (300-600 nm) for catalysis. In the reaction, the enzyme converts cyclobutyl pyrimidine dimers in the DNA to monomer pyrimidines. The enzyme has an apparent monomer molecular weight of 40,000 and tends to form aggregates. The pH optimum of 7.2 and absence of a requirement for metal ions are similar to the requirements of the Escherichia coli enzyme; however, the ionic strength optimum of 0.05 is much lower than those for other photoreactivating enzymes. The demonstration that human cells possess photoreactivating enzyme implies that a direct test by photoreactivation may be made of the role of pyrimidine dimers in the induction of abnormal cell growth.

Xeroderma pigmentosum cells contain low levels of photoreactivating enzyme

Fibroblasts from patients with xeroderma pigmentosum contain low levels of photoreactivating enzyme in comparison to normal cells. Levels vary from 0 (line 1199) to 50 (line 1259) percent of normal. The depressed enzyme levels are not an artifact of low growth rate, age of cell donor, cell culture conditions, assay conditions, the presence of inhibitors, or mycoplasma contamination. We show that human fibroblasts can monomerize pyrimidine dimers in vivo.

Use of enzymatic assay to evaluate UV-induced DNA repair in human and embryonic chick fibroblasts and multinucleate heterokaryons derived from both

A sensitive enzymatic assay has been utilized to monitor repair of UV-induced damage to DNA in primary human and embryonic chick cells and in multinucleate heterokaryons artificially derived from both. The assay exploits the unique ability of a purified repair endonuclease to attack UV-irradiated DNA at sites containing pyrimidine dimers. These nuclease-susceptible sites are subsequently observed as single-strand scissions by velocity sedimentation in alkaline sucrose gradients. Incubation of UV-damaged cultures followed by extraction and enzymatic analysis of the radioactively labeled DNA enables one to trace the disappearance of such sites in vivo and hence to monitor endogenous repair activity. When UV-irradiated human cells are incubated in the dark, the curve for site removal exhibits a two-phase exponetial decline; i.e. there exists a fast component responsible for elimination of 60% of the initial damage and a second one approximately 7 times slower in rate. The removal of sites is not further enhanced by exposing cells to blacklight during post-UV incubation. Conversely, UV-damaged chick cells rid their DNA of all nuclease-susceptible sites rapidly (i.e. at an exponential rate approximately 13 times faster than the fast component of site removal in human cells) when incubated under blacklight but not when kept in the dark. These data indicate the presence in human and embryonic chick cells of distinct enzymatic mechanisms for the elimination of dimer-containing sites. Wheneras human fibroblasts rely heavily on a light-independent process, excision-repair, chick fibroblasts possess a light-dependent mechanism, presumably photoenzymatic repair. Advantage has been taken of the contrasting repair properties of the human and embryonic chick fibroblasts to evaluate the extent to which each can assist the other in the removal of UV-induced damage from its DNA. The two cell types were fused to form giant human/chick heterokaryons containing a number of intact nuclei from both strains. Experimental conditions were selected so that UV-induced damage resided only in DNA foreign to the repair enzymes under study. Our results strongly suggest that repair enzyme(s) coded for by either fusion partner can remove dimer-containing sites from the DNA of the other with an efficiency comparable to that attained when acting on its own DNA in unfused, parental cells. Further, the light-requiring repair process supplied by the chick is more proficient at operating on these sites in human DNA than is excision-repair, the parallel mechanism available to human cells for this purpose.

Photoreactivation and excision repair of ultraviolet radiation-injured DNA in primary embryonic chick cells

Primary embryonic chick cells have been evaluated on the basis of their capacity to repair photochemical lesions produced in the deoxyribonucleic acid (DNA) by ultraviolet (UV) radiation. The fate of one prominent class of UV photoproducts, cyclobutane pyrimidine dimers, was monitored by an in vitro enzymatic assay. UV-irradiated cultures were incubated for prescribed times after which their damaged, radioactive-labeled DNA was extracted and exposed to a purified UV endonuclease selectively active toward sites altered by dimer formation. Single-strand scissions specifically introduced by the enzyme treatment and, therefore, the dimer-containing sites remaining in the DNA were quantified retrospectively by velocity sedimentation in alkaline sucrose. When the chick fibroblasts were incubated in black light, essentially all nuclease-susceptible sites rapidly disappeared from the UV-damaged DNA. In sharp contrast, incubation of the irradiated cultures in total darkness severely impeded the metabolic machinery responsible for site elimination. A substantial amount of UV-stimulated DNA repair synthesis was also detected in the chick cells by conventional techniques involving isopyknic centrifugation and autoradiography. However, the UV photoproducts triggering this indicator of excision repair were probably not dimers since incubation of the irradiated cultures in the light rather than in the dark did not lead to a diminution in the extent of repair synthesis. By these criteria of DNA repair, it appears that embryonic chick cells primarily rely on a highly proficient, light-requiring mechanism, presumably enzymatic photoreactivation, for dimer elimination but also possess a light-independent, excision-type process to cope with other, as yet unidentified, photochemical defects.

Photoreactivation of a cytoplasmic virus

Ultraviolet light-inactivated frog virus 3 is efficiently photoreactivated by chick embryo cells. A cellular enzyme is presumably responsible for this repair of viral deoxyribonucleic acid, for the phenomenon is insensitive to an inhibitor of protein synthesis and is not seen in mammalian cells that are known to lack photoreactivating enzyme. Since frog virus 3 is a cytoplasmic virus, functionally significant amounts of photoreactivating enzyme are probably present in the cytoplasm of chick embryo cells.