Determination of the mechanism of retrotransfer by mechanistic mathematical modeling.

Two mathematical models to elucidate the mechanism of retromobilization (or retrotransfer), that is, the ability of conjugative plasmids to mobilize genes into the cell containing the conjugative plasmid, were developed. This study deals with retromobilization of nonconjugative plasmids (Tra-Mob+). Plasmid transfer was modeled by two mass action models. The first is based on the hypothesis that retromobilization of the Tra-Mob+ vector occurs in one step, by means of the pilus formed by the Tra+ plasmid in the original host. In the second model, retromobilization is considered to be a two-step process involving two transfer events. The first step involves the transfer of the Tra+ plasmid from the recipient cell to the donor of the nonconjugative vector, and during the second encounter the nonconjugative vector is mobilized toward the recipient. Since the relationships between the number of transconjugants and the number of recipients for the two models are different, filter matings were performed for short time periods with different initial densities of the recipient population. Comparison of the numbers of transconjugants with the results of the mathematical equations confirmed the hypothesis that retromobilization is a one-step conjugation process.     

Prevalence of coaggregation reactions among chicken lactobacilli

L. VANDEVOORDE, H. CHRISTIAENS AND W. VERSTRAETE. 1992. Interbacterial adherence was frequently encountered among chicken lactobacilli. Fourteen of 45 combinations involving nine adhering strains were shown to be coaggregative. The coadherence mechanism was mediated by complementary heat- and sonication-sensitive cell surface structures. It was shown that intrageneric adherence enabled lactobacilli to maintain higher numbers in fed-batch reactors simulating the gastrointestinal tract. The mechanism of coaggregation can substantially increase the colonization potential of lactobacilli in environments with short residence times.     

Manganese as a controlling factor in mixed cultures of Lactobacillus plantarum and enterobacteriaceae

Addition of manganese, at levels of 50 ppm, to a liquid growth medium simulating adverse silage conditions had no effect on the growth or on the fermentation pattern of Enterobacter cloacae and Proteus vulgaris. Yet, the manganese strongly enhanced the growth of Lactobacillus plantarum. Co-cultures of L. plantarum and E. cloacae or P. vulgaris were, by addition of manganese ions, significantly altered in the favour of the former. This finding can be of use in mixed cultures where Enterobacteriaceae act as spoiler microorganisms.     

In vitro synthesis of lectins in cell-free extracts from dry wheat and rye embryos

Cell-free extracts from dry wheat (Triticum vulgare L.) and rye (Secale cereale L.) embryos do not synthesize their corresponding lectins when incubated under conditions optimalized for translation of either exogenous or endogenous mRNA. Only when the extracts are prepared and incubated in the complete absence of sulphydryl containing compounds lectins are synthesized in vitro. Since immunoprecipitation techniques could not demonstrate unequivocally the presence of lectin among the cell-free translation products a specific affinity purification procedure has been developed in order to proof the presence of stored lectin mRNAs in dry wheat and rye embryos.Abbreviations DTT dithiothreitol - SDS-PAGE sodium dodecyl sulphate polyacrylamide gel electrophoresis - TCA trichloracetic acid     

The effect of electrical gradients on current fluctuations and impedance recorded fromNecturus gallbladder

Summary Transepithelial current fluctuations were recorded inNecturus gallbladder, clamped at negative as well as positive potentials up to 64 mV. With NaCl-Ringer's (+10mm TAP) on both sides a mucosa-negative potential enhanced the relaxation noise component, present at zero potential, and produced peaking in the power spectrum at potentials above –36mV. Concomitantly at these potentials an inductive as well as a capacitive low-frequency feature appeared in the impedance locus. Clamping at positive potentials of 18 mV suppressed the relaxation noise component. At potentials above 51mV the spectral values increased predominantly at low frequencies. In this case the power spectrum showed only a 1/f noise component. The experiments confirm the previous finding that a K⁺ efflux through fluctuating apical K⁺ channels exists under normal conditions. With serosal KCl-Ringer's the initial Lorentzian component was enhanced at negative but suppressed at positive potentials. The increase at negative potentials was less pronounced than in experiments with NaCl-Ringer's on both sides, indicating saturation of the fluctuating K⁺ current component. With mucosal KCl-Ringer's a negative potential depressed the initial relaxation noise component, whereas it was enhanced at +18 mV clamp potential. In the latter case an additional Lorentzian component became apparent at higher frequencies. At potentials of 36 mV and above the low-frequency Lorentzian disappeared whereas the corner frequency of the high-frequency component increased. The latter experiments demonstrate that the relaxation noise component inNecturus gallbladder consists of two superimposed Lorentzians. As the relaxation times of these two components behave differently under an electrical field, there may exist two different types of K⁺ channels. It is demonstrated that peaking in the plateau of power spectra can be explained by frequency-dependent attenuation effects, caused by a polarization impedance.     

Lipoxygenase activity in apples in relation to storage and physiological disorders

Different methods for the preparation of active lipoxygenase (LOX) extracts from apples were compared. Highest activities were obtained using a 0.25 M phosphate buffer (pH 7) containing 1% Triton X-100 and 10^?2 M metabisulphite as extraction solvent. LOX activity during storage was investigated in the core, flesh, and peel. Activity was always highest in the core and peel. On storage, activity was increased in each part of the fruit but especially in the core and peel. Increase in LOX preceded the browning of the core. LOX may be responsible for the browning and may be concerned in the induction of superficial scald.     

Comparison of maximum cell yield and maintenance coefficients in axenic cultures and activated sludge communities

Comparison of the equations that describe the relationship between the maximum cell yield coefficient, the maintenance coefficient, and the specific growth rate at steady-state conditions revealed that the equations used for axenic cultures are congruent with those commonly used for mixed-culture system such as activated sludge. A unified basis was proposed. The expression of the yield and maintenance coefficients in carbon units according to the unified basis permitted one to evaluate literature data on both axenic and mixed-culture systems. From this it appears that the maximum cell yield ranges from 0.50–0.80 (mg biomass carbon formed/mg substrate carbon used) for both axenic and mixed systems. However, the maintenance coefficient (mg substrate C/mg biomass C·hr) for the axenic cultures was between 0.010 and 0.100, but for activated sludge communities it was between 0.001 and 0.010. Microorganisms were isolated from sludge communities with these apparently low maintenance requirements and grown axenilly. Their maintenance coefficients but not their maximum yield coefficients decreased with decreasing specific growth rates. The consequences of this finding with regard to species selection in mixed-culture systems and the concept of cellular maintenance requirement are discussed.     

Further studies on polyploid amphibians

The manner in which centromere regions of mitotic chromosomes are distributed with respect to the age of their DNA was studied. Cells of the Indian deer, Muntiacus muntjak, were grown in the presence of bromodeoxyuridine (BrdU) for two generations and stained with the fluorescent dye Hoechst 33258. Chromatids containing granddaughter DNA appear dim when compared with those containing grandparental DNA. The frequencies of the various anaphase patterns of bright and dim centromere regions were binomially distributed, indicating random distribution of chromatids with respect to the age of their DNA templates.