Plant-microbial interaction under gnotobiotic conditions: A scanning electron microscope study

Inoculation of canola seeds withPseudomonas putida GR12-2 stimulates root elongation under gnotobiotic conditions. Transformation ofP. putida GR12-2 with the broad-host-range plasmid pGSS15 abolishes the enhancement of root elongation. With scanning electron microscopy it was found that both transformed and nontransformedP. putida GR12-2 are capable of binding to canola seed coats. In addition, it was observed that 4 days after the initial inoculation the roots of bothP. putida GR12-2- and GR12-2/pGSS15-treated seedlings were free of adhering bacteria despite the fact that it was subsequently shown that both bacterial strains are capable of binding to roots. Thus, adhesion to roots is not necessary for the initial phase of enhanced root elongation that is induced byP. putida GR12-2 under gnotobiotic conditions.     

Identification of the products and nucleotide sequences of two regulatory genes involved in the exogenous induction of phosphoglycerate transport in Salmonella typhimurium.

We describe the determination of the nucleotide sequence of two genes (pgtB and pgtC) contained within the 3.4-kilobase DNA segment sandwiched between the transporter gene, pgtP, and the regulatory gene, pgtA. These two genes are involved in the regulation of expression of phosphoglycerate transport in Salmonella typhimurium. The sequence indicates the presence of two large open reading frames, potentially coding for two polypeptides of 397 and 593 amino acid residues. The two gene products were identified by using the bacteriophage T7 RNA polymerase-T7 promoter coupled system of Tabor and Richardson, and the observed apparent mass of 45 and 69 kilodaltons correlated well with the respective open reading frames. The cellular location of these two polypeptides was directly determined, and the polypeptides were found to be associated with the membrane. Although overall these polypeptides appear to be hydrophilic, there is one hydrophobic transmembrane segment in the smaller polypeptide and four such segments in the larger polypeptide which can account for their association with the membrane. In the accompanying paper, we present genetic evidence that pgtB and pgtC genes are involved in the induction of the pgtP expression by modulating derepressor activity.     

Fluorometric detection of the bilayer-to-hexagonal phase transition in liposomes

We have used the fluorescent probe N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) to detect the bilayer-to-hexagonal phase transition. The fluorescence intensity of the probe was found to increase during the bilayer-to-hexagonal transition. The bilayer-to-hexagonal transitions of various types of phosphatidylethanolamine or cardiolipin measured by this method are consistent with results obtained by differential scanning calorimetry. To establish this method for wider use, agents known to alter the bilayer-to-hexagonal transition were examined, and the results are comparable with the published data. The added advantage of this fluorometric method over other currently available techniques is that it is applicable not only for aggregated lipid samples but also for dilute liposome suspensions. This is especially important when one of the components of the system under study can partition between lipid and aqueous phase. Since NBD is located near the headgroup region of the bilayer, it most likely detects the change of the environment surrounding that region. On the basis of our present study, it appears that NBD-PE is sufficiently sensitive to detect bilayer-to-hexagonal phase transition.     

Termination of gonadal refractoriness in Turkish hamsters, Mesocricetus brandti

The Turkish hamster (Mesocricetus brandti) is a photoperiodic species. In this investigation, we characterized the photoperiodic requirements for termination of gonadal refractoriness, defined as the inability of the animal to respond to short-day treatment with gonadal regression. Paired testes weights were reduced to less than 20% of their original weight by 10 wk of 12L:12D treatment. This was followed by spontaneous testicular recrudescence (completed by Week 25 of 12L:12D treatment), the overt indication of refractoriness to short photoperiods. Next, the period of long-day exposure sufficient for termination of refractoriness was determined. Refractory males were exposed to 16L:8D for 5 to 20 wk. Ten weeks of 16L:8D treatment was enough for the animals to regain the sensitivity to a second challenge of 12L:12D treatment. Fifteen weeks of 20L:4D or 16L:8D terminated refractoriness in female Turkish hamsters; 20L:4D therefore was not interpreted as a short day by refractory hamsters. This was unexpected because in photosensitive animals this photoperiod acts like a short day, causing gonadal regression. These results suggest that Turkish hamsters are similar to Syrian hamsters in that both species require two or more months of long days in summer to recover sensitivity to the short days of the following fall.     

Optimal substrate feeding policy for a fed batch fermentation with substrate and product inhibition kinetics

The optimal substrate feeding policy for the fed batch fermentation which is governed by product and substrate inhibited kinetics is presented. The conjunction point between nonsingular and singular arcs and the feeding policy along the singular arc are derived analytically in terms of the concentrations of substrate and product and the liquid volume. Thus, it is possible to determine the feeding rate by monitoring the state variables (i.e., closed loop control). As a specific example, an optimization study of the fed batch fermentation for ethanol production by Saccharomyces cerevisiae is presented. It is shown that the optimal feeding patterns are heavily dependent upon the initial conditions. The point selectivity provides the guideline for predicting the optimal feeding patterns and explaining the results of rigorous mathematical analysis.