Temperature-sensitive initiation of DNA replication in a mutant of Escherichia coli K12

Summary A mutant of E. coli K12 appears to be temperature-sensitive in the process of initiation of DNA replication. After a temperature shift from 33 to 42°C, the amount of residual DNA synthesis (Fig. 1) and the number of residual cell divisions (Figs. 2,4) indicate that rounds of DNA replication in process are completed, but new rounds cannot be initiated. Following the alignment of chromosomal DNA by amino acid starvation at 33° C no residual DNA synthesis at 42°C takes place (Fig. 5). When the temperature is lowered to 33°C after a period of inhibition at 42°C, the following observations are made: 1. DNA replication resumes and proceeds synchroneously, (Figs. 7, 8a), 2. cells start to divide again only after a lag period of about 1 hour 3. a temporary increase in cell volume is correlated with the frequency of initiation of DNA synthesis (Fig. 8a, b). In a lysogenic mutant strain prophage is inducible; with all bacteriophages tested, replication of phage DNA is not inhibited at 42°C.     

The fate of newly made DNA in Escherichia coli mutants thermosensitive in DNA synthesis

Summary E. coli mutants exist in which DNA synthesis is thermosensitive. In one class of these mutants DNA synthesis stops immediately if a critical temperature (42°C) is reached. When DNA replication in such mutants is followed by ³H thymidine incorporation at 33°C, it is found that 1. only the newly made DNA is degraded at 42°C, 2. the discontinuously replicated DNA is lost predominantly at 42°C, 3. 1–3% of the chromosomal DNA is rendered acid soluble at 42°C without concomitant loss of viability of the cells at 33°C.Replication of phage DNA is inhibited in the same mutant at 42°C. However, when DNA synthesis is followed in infected cells at 33°C it is found that 1. no degradation of specific DNA seems to occur at 42°C in the early phase of infection, 2. replicating DNA molecules in the late phase of infection are completed at 42°C before DNA synthesis comes to a halt.     

Analysis of relaxation time data from a low-resolution 1H-NMR-pulse-spectrometer

Systematic investigations have been undertaken in order to evaluate the potential of low resolution NMR for characterization of biological tissue (in vitro) during early post mortem period. Test measurements from corn-oil samples are compared with computer simulated data. Furthermore, time-after-excision dependence of mouse-liver tissue is presented using the in vitro protocol developed in our laboratory. Quantitative data from biexponential model fit are shown and results are discussed in terms of the "best guess" model.     

ban operon of bacteriophage P1. Mutational analysis of the c1 repressor-controlled operator

The repressor of bacteriophage P1, encoded by the c1 gene, represses the phage lytic functions and is responsible for maintaining the P1 prophage in the lysogenic state. The c1 repressor interacts with at least 11 binding sites or operators widely scattered over the P1 genome. From these operators, a 17 base-pair asymmetric consensus sequence, ATTGCTCTAATAAATTT, was derived. Here, we show that the operator, Op72 of the P1ban operon consists of two overlapping 17 base-pair sequences a and b forming an incomplete palindrome. Op72a matches the consensus sequence, whereas Op72b contains two mismatches. The evidence is based on the sequence analysis of 27 operator mutants constitutive for ban expression. They were identified as single-base substitutions at positions 2 to 10 of Op72a (26 mutants) and at position 8 of Op72b (one mutant). We conclude from gel retardation and footprinting studies that two repressor molecules bind to the operator and that positions 4, 5 and 7 to 10 of the operator play an essential role in repressor recognition.     

Dynamics of small autocatalytic reaction networks—I. bifurcations,permanence and exclusion

Catalysis in replication networks has become an important issue in biophysics and other areas of biology. Examples are RNA catalysis, idiotype recognition in the immune response and dynamical models of Maynard-Smith games in sociobiology. Chemical reaction networks describing catalysed, template-induced reproduction of three species are analysed in full generality. The nine-dimensional parameter space is reduced to three relevant angular coordinates which determine completely the phase portraits (PPs) and the bifurcation patterns. All cases are classified and all generic as well as most of the nongeneric transitions are listed and described. This paper has been reproduced directly from disc using a LA-T_EX system.     

Construction and packaging of pseudotype retrovirus containing human N—ras cDNA antisense sequence and its biological effects on human hepatoma cells

N-ras is one of the transforming genes in human hepatic cancer cells.It has been found that N-ras was overexpressed at the mRNA and protein level in hepatoma cells.In order to explore the biological roles of N-ras in human hepatic carcinogenesis and the potential application in control of cancer cell growth,a preudotype retrovirus containing antisense sequence of human N-ras was constructed and packaged.A recombinant retrovirus vector containing antisense or sense sequences of N-ras cDNA was constructed by pZIP-NeoSV(X)1.The pseudotype virus was packaged ang rescued by transfection and infection in PA317 and ψ 2 helper cells.It has been demonstrated that the pseudotype retrovirus containing antisense N-ras sequence did inhibit the growth of human PLC/PRF/5 hepatoma cells accompanied with inhibition of p21 expression,while the retrovirus containing sense sequence had none.The pseudotype virus had no effect on human diploid fibroblasts.     

Alveolar slope and dead space of He and SF6 in dogs: comparison of airway and venous loading

Series (Fowler) dead space (VD) and slope of the alveolar plateau of two inert gases (He and SF6) with similar blood-gas partition coefficients (approximately 0.01) but different diffusivities were analyzed in 10 anesthetized paralyzed mechanically ventilated dogs (mean body wt 20 kg). Single-breath constant-flow expirograms were simultaneously recorded in two conditions: 1) after equilibration of lung gas with the inert gases at tracer concentrations [airway loading (AL)] and 2) during steady-state elimination of the inert gases continuously introduced into venous blood by a membrane oxygenator and partial arteriovenous bypass [venous loading (VL)]. VD was consistently larger for SF6 than for He, but there was no difference between AL and VL. The relative alveolar slope, defined as increment of partial pressure per increment of expired volume and normalized to mixed expired-inspired partial pressure difference, was larger by a factor of two in VL than in AL for both He and SF6. The He-to-SF6 ratio of relative alveolar slope was generally smaller than unity in both VL and AL. Whereas unequal ventilation-volume distribution combined with sequential emptying of parallel lung regions appears to be responsible for the sloping alveolar plateau during AL, the steeper slope during VL is attributed to the combined effects of continuing gas exchange and ventilation-perfusion inequality coupled with sequential emptying. The differences between He and SF6 point at the contributing role of diffusion-dependent mechanisms in intrapulmonary gas mixing.