Biomedical subjects

YC Liu

Publications and source records attributed to YC Liu.

8 recordsLinked to original sources

PubMed1998

Growth rate regulation of rRNA content of a marine synechococcus (Cyanobacterium) strain

The relationship between growth rate and rRNA content in a marine Synechococcus strain was examined. A combination of flow cytometry and whole-cell hybridization with fluorescently labeled 16S rRNA-targeted oligonucleotide probes was used to measure the rRNA content of Synechococcus strain WH8101 cells grown at a range of light-limited growth rates. The sensitivity of this approach was sufficient for the analysis of rRNA even in very slowly growing Synechococcus cells (&mgr; = 0.15 day-1). The relationship between growth rate and cellular rRNA content comprised three phases: (i) at low growth rates (< approximately 0.7 day-1), rRNA cell-1 remained approximately constant; (ii) at intermediate rates ( approximately 0. 7 - 1.6 day-1), rRNA cell-1 increased proportionally with growth rate; and (iii) at the highest, light-saturated rates (> approximately 1.6 day-1), rRNA cell-1 dropped abruptly. Total cellular RNA (as measured with the nucleic acid stain SYBR Green II) was well correlated with the probe-based measure of rRNA and varied in a similar manner with growth rate. Mean cell volume and rRNA concentration (amount of rRNA per cubic micrometer) were related to growth rate in a manner similar to rRNA cell-1, although the overall magnitude of change in both cases was reduced. These patterns are hypothesized to reflect an approximately linear increase in ribosome efficiency with increasing growth rate, which is consistent with the prevailing prokaryotic model at low growth rates. Taken together, these results support the notion that measurements of cellular rRNA content might be useful for estimating in situ growth rates in natural Synechococcus populations.

Journal Article↗

PubMed1997

Sedimentation Velocity and Potential in a Dilute Suspension of Charged Composite Spheres

The sedimentation of a charged composite particle composed of a solid core and a surrounding porous shell in an electrolyte solution is analytically studied. In the solvent-permeable and ion-penetrable porous surface layer of the particle, idealized hydrodynamic frictional segments with fixed charges are assumed to distribute at a uniform density. The equations which govern the ionic concentration distributions, the electric potential profile, and the fluid flow field inside and outside the surface layer of a charged composite particle migrating in an unbounded solution are linearized assuming that the system is only slightly distorted from equilibrium. Using a perturbation method, these linearized equations are solved for a composite sphere with the charge densities of the rigid core surface and of the surface layer as the small perturbation parameters. An analytical expression for the settling velocity of the composite sphere in closed form is obtained from a balance among its gravitational, electrostatic, and hydrodynamic forces. The result demonstrates that the presence of the fixed charges in the composite sphere slows down its settling velocity relative to that of an uncharged one. A closed-form formula for the sedimentation potential in a dilute suspension of identical charged composite spheres is also derived by using the requirement of zero net electric current. The Onsager reciprocal relation is found to be satisfied between sedimentation and electrophoresis. It is shown that spherically-symmetric "neutral" composite particles (bearing no net charge) can undergo electrophoresis, induce sedimentation potential, and experience a smaller settling velocity relative to corresponding uncharged particles. The direction of the electrophoretic velocity or the induced potential gradient is determined by the fixed charges in the porous surface layers of the particles. In the limiting cases, the analytical solutions describing the sedimentation velocity and sedimentation potential (or electrophoretic mobility) for charged composite spheres reduce to those for charged solid spheres and for charged porous spheres. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗

PubMed1997

The Electric Conductivity of Dilute Suspensions of Charged Porous Spheres

The effective electric conductivity of a dilute suspension of polyelectrolyte molecules or charged flocs in an electrolyte solution is analytically studied. The model used for the particles is a porous sphere in which the density of hydrodynamic frictional segments, and therefore also that of the fixed charges, is constant. The equations which govern the electrochemical potential distributions of ionic species and the fluid flow field inside and outside a porous particle migrating in an unbounded solution are linearized assuming that the system is only slightly distorted from equilibrium. Using a perturbation method, these linearized equations are solved for a porous sphere in a uniform applied electric field with the density of the fixed charges as the small perturbation parameter. An analytical expression for the effective conductivity of a dilute suspension of identical charged porous spheres is obtained from the average electric current density calculated using the solution of electrochemical potential distributions of the ions. The result demonstrates that the presence of the fixed charges in the porous particles can lead to an augmented or a diminished electric conductivity of the suspension relative to that of a corresponding suspension of uncharged porous particles, depending on the characteristics of the electrolyte solution and the suspending particles. When the anionic and cationic diffusion coefficients of a symmetric electrolyte are the same, the correction for the effect of the fixed charges of the particles on the electric conductivity of the suspension is proportional to the square of the fixed charge density. Copyright 1997Academic Press

Journal Article↗

PubMed1996