Orientational dynamics of magnetotactic bacteria in Earth’s magnetic field—a simulation study

We investigate through simulations the phenomena of magnetoreception to enable an understanding of the minimum requirements of a fail-safe mechanism, operational at the cellular level, to sense a weak magnetic field at ambient temperature in a biologically active environment. To do this, we use magnetotactic bacteria (MTB) as our model system. The magnetic field sensing ability of these bacteria is due to the presence of magnetosomes, which are internal membrane-bound organelles that contain an iron-based magnetic mineral crystal. These magnetosomes are usually found arranged in a chain aligned with the long axis of the bacterial body. This arrangement yields an overall magnetic dipole moment to the bacterial cell. To simulate this orientation process, we set up a rotational Langevin stochastic differential equation and solve it repeatedly over appropriate time steps for isolated spherical shaped MTB as well as for a more realistic model of spheroidal MTB with flagella. The orientation process appears to depend on shape parameters with spheroidal MTB showing a slower response time compared to spherical MTB. Further, our simulation also reveals that the alignment to the external magnetic field is more robust for an MTB when compared to single magnetosome. For the simulation involving magnetosomes, we include an extra torque that arises from the twisting of an attachment tether and enhance the viscosity of the surrounding medium to mimic intracellular conditions in the governing Langevin equation. The response time of alignment is found to be substantially reduced when one includes a dipole interaction term with a neighboring magnetosome and the alignment becomes less robust with increase in inter dipole distance. The alignment process can thereby be said to be very sensitively dependent on the distance between magnetosomes. Simulating the process of alignment between two neighboring magnetosomes, both in the absence and presence of an ambient magnetic field, we conclude that alignment between these dipoles at the distances typical in an MTB is highly probable and it would be the locked unit that responds to changes in the external magnetic field.     

Biosynthesis of branched-chain amino acids in yeast: effect of carbon source on leucine biosynthetic enzymes.

The three enzymes in the leucine biosynthetic pathway of yeast do not exhibit coordinate repression and derepression in response to the carbon source available in the culture medium. Growth in an acetate medium results in derepression of the first enzyme in the pathway, alpha-isopropylmalate synthase, and repression of the second two enzymes, alpha-isopropylmalate isomerase and beta-isopropylmalate dehydrogenase, relative to the levels found in glucose-grown cells. The role of endogenous leucine pools as a mediator of these differences was investigated. The leucine pools did not differ significantly between acetate-grown and glucose-grown cells. However, an elevated endogenous leucine pool, caused by exogenous leucine in the growth medium, did decrease the rate of decay of alpha-isopropylmalate synthase activity observed when acetate-grown cells were shifted to glucose. Evidence is provided suggesting that an elevated endogenous leucine pool may increase the in vivo stability of alpha-isopropylmalate synthase under several different conditions. Studies on the kinetics of alpha-isopropylmalate synthase decay in vivo and sensitivity to leucine inhibition indicate that there are two classes of the enzyme in acetate-grown yeast cells.     

A dual role of Cdk2 in DNA damage response

Once it was believed that Cdk2 was the master regulator of S phase entry. Gene knockout mouse studies of cell cycle regulators revealed that Cdk2 is dispensable for S phase initiation and progression whereby Cdk1 can compensate for the loss of Cdk2. Nevertheless, recent evidence indicates that Cdk2 is involved in cell cycle independent functions such as DNA damage repair. Whether these properties are unique to Cdk2 or also being compensated by other Cdks in the absence of Cdk2 is under extensive investigation. Here we review the emerging new role of Cdk2 in DNA damage repair and also discuss how the loss of Cdk2 impacts the G₁/S phase DNA damage checkpoint.