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Biomedical subjects

R K Chan

Publications and source records attributed to R K Chan.

83 records · Page 5Linked to original sources
PubMed1977

Recovery of Saccharomyces cerevisiae mating-type a cells from G1 arrest by alpha factor.

Mating-type a cells of the yeast Saccharomyces cerevisiae that had been specifically arrested in the G1 phase of the cell cycle by alpha factor, an oligopeptide pheromone made by alpha cells, recovered and resumed cell division after a period of inhibition which was dependent on the concentration of alpha factor used. These treated a cells were more resistant to alpha factor than untreated a cells, but lost their resistance upon further cell division. However, cells arrested for 6 h were no more resistant to alpha factor than cells arrested for only 2.5 h. Mating-type a strains could inactivate or remove alpha factor from the culture fluid, but two a sterile (nonmating) mutants and an a/alpha diploid strain could not. These results suggest that a cells have a mechanism, which may involve uptake or inactivation of alpha factor, for recovering from alpha factor arrest. However, the results do not distinguish between a recovery mechanism which is constitutive and one which is induced by alpha factor. The loss of alpha factor activity during recovery appeared to be primarily cell contact mediated, although an extracellular, diffusible inhibitor of alpha factor that is labile or that functions stoichiometrically could not be ruled out.

Culture Media↗
PubMed1976

Specialized transduction by bacteriophage P22 in Salmonella typhimurium: genetic and physical structure of the transducing genomes and the prophage attachment site.

P22pro-1 and P22pro-3 are specialized transducing derivatives of phage P22 that carry the proA and proB genes of Salmonella typhimurium. These genes lie immediately adjacent to the prophage attachment site on the bacterial chromosome. By examining DNA heteroduplexes in the electron microscope, we found that DNA molecules from P22pro-1 and P22pro-3 each contain a substitution which adds length to the composite genome making the intracellular replicated genome too long to fit into a single phage particle. In this respect, and in many of their biological properties, the proline-transducing phages resemble P22Tc-10, another specialized transducing phage with an oversize, intracellular replicated genome which carries a tetracycline-resistance determinant from an R-factor.--Unlike P22Tc-10, however, P22pro-1 and P22pro-3 fail to integrate normally during lysogenizing infections, even when provided with all known integration functions. These results suggest that the proline substitutions have created a defect in the phage attachment site and suggest that the Campbell model for the formation of specialized transducing phages is applicable to phage P22 with the additional feature that oversize genomes can be produced and propagated.--A physical and genetic map of the P22 genome near the prophage attachment site was constructed which shows that the insertion from the R-factor in P22Tc-10 is not at the attachment site: it is therefore unlikely that P22Tc-10 was formed in an abnormal prophage excision event as envisioned in the Campbell model, but was instead the result of a direct translocation from the R-plasmid to P22.

Chromosome Mapping↗
PubMedDate not supplied

Effects of chronic captopril treatment on the electrical-microstimulation-induced blood pressure changes and electrophysiological properties of cardiovascular neurons in the rostral ventrolateral medulla of the spontaneously hypertensive rat.

The effects of chronic treatment with captopril, an angiotensin-converting enzyme inhibitor, on various properties of the rostral ventrolateral medulla (RVL) were studied in spontaneous hypertensive rats (SHR) given captopril (orally at a dose of 30 mg/kg) or vehicle from the age of 4-5 weeks to 19-20 weeks. In SHR, such a treatment with captopril resulted in an arterial blood pressure similar to that of normotensive Wistar Kyoto rats (WKY). Upon electrical microstimulation of the RVL, the minimal threshold current required to elicit a pressor response in captopril-treated SHR was significantly higher than that observed in SHR. Furthermore, analysis of the discharge pattern of RVL spinal projecting cardiovascular neurons showed a significant difference between captopril-treated and untreated SHR with respect to the proportion of double discharge units and the regularity of single discharge units. These properties of the RVL in captopril-treated SHR were, however, similar to those of WKY. Upon withdrawal of electrical microstimulation, the secondary sustained pressor response characteristic of SHR, but not of WKY, persisted in captopril-treated SHR at a smaller magnitude and a shorter duration. These results indicate that the normalization of arterial blood pressure in SHR upon chronic oral treatment with angiotensin-converting enzyme inhibitor is associated with changes in properties of RVL cardiovascular neurons towards those of WKY. This suggests that brain angiotensin and RVL cardiovascular neurons play interactive roles in the development of hypertension in SHR.

Angiotensin II↗
PubMedDate not supplied

Evaluation of a safe-sex intervention programme among sex workers in Singapore.

The purpose of this study was to evaluate the effects of a brief intervention programme on STD knowledge, condom use and gonorrhoea incidence among sex workers in Singapore. A controlled before-and-after study design with non-random assignment of sex workers was used, supplemented by multivariate analysis to adjust for baseline differences. Control (n = 221, denoted C1) and experimental (n = 221, denoted E1) groups were interviewed on 2 occasions 3 months apart. Two supplementary groups were interviewed once each at the end of the 3-month period (n = 145 who had received the intervention and n = 151 who had not). Basic knowledge of STD symptoms and HIV transmission was high in all groups. There were misconceptions about casual transmission of HIV which improved dramatically at the second interview for group E1 (from 37-56% correct responses to 82-90%). Overall condom use was high (about 75%) and did not change after the intervention. Gonorrhoea rates were correspondingly low (0.4 episodes/worker/year) and also did not change. This brief intervention improved the STD knowledge of sex workers. However, behaviour as measured by reported condom use and gonorrhoea incidence did not change. Implications for future intervention programmes are discussed.

Adult↗