Biological cell is the smallest system in which energy is transformed and information is processed autonomously. This complex and fascinating system is in the far from equilibrium state, and hence can manifest self-excited oscillations. Here we will describe rhythmic locomotion in amoeboid cells, oscillatory responses to chemical stimuli and rhythmic morphogenesis. Their biological and biophysical implications are discussed.
In the slug stage of cellular slime molds Dictyostelium discoideum, the prestalk cells occupy the anterior 1/4, and among the remaining posterior prespore cells there are about 10% anterior-like cells which have the same staining property as the prestalk cells. This slug differentiation pattern is well regulated independent of the slug size. Further, the pattern is basically restored if a part of the slug is excised. This survey reviews several such models that may explain the formation and the regulation capability of the differentiation pattern. The models treated are classified into the following three categories. 1) Positional information is established first, and then the differentiation follows accordingly; 2) The cytodifferentiation occurs through the balance of the labor division, and the cells sort out to form the pattern; 3) The pattern is thought to be in a dynamical equilibrium, i.e., the prestalk and the anteriorlike cells interchange incessantly while keeping the whole pattern.
Photoreactivation-a reduction in the effect of ultraviolet irradiation by subsequent exposure to longer wavelengths-stems from at least two different kinds of process. The first is direct, non-enzymatic short-wavelength reactivation and photoenzymemediated repair of ultraviolet radiation damage to DNA, while the second (indirect photoreactivation) is an enhancement of light-independent repairs due to physiological changes induced in cells by light. Enzymatic photoreactivation is an enzymatic process in which damage induced by ultraviolet radiation is repaired with light of longer wavelengths (300-500nm). It is achieved by a photoreactivating enzyme. On the molecular level, photoreactivating enzyme splits pyrimidine dimers in DNA into the constituent pyrimidines. Thus, photoreactivating enzyme has biological importance because of its role in one mode of DNA repair and is also of physico-chemical interest as an enzyme which utilizes light for its catalytic action. Purfied Escherichia coli photoreactivating enzyme is a single polypeptide of Mr 49.000 which has absorption peaks at 280 and 380nm and, upon denaturation, releases a chromophore that has the properties of flavin adenine dinucleotide, indicating that flavin is an intrinsic chromophore of the enzyme.
Mitochondrial F1-F0 complex catalyzes ATP synthesis coupled with proton translocation down the electrochemical potential gradient (ΔμH+) across the inner membrane. F1, the catalytic sector of the complex, can be detached from the membrane to be a soluble protein. Recently, we found that soluble F1 catalyzed ATP synthesis from medium ADP and Pi in the presence of dimethylsulfoxide (DMSO). This finding indicates that energy input of ΔμH+ is not essential to covalent binding of ADP and Pi to form ATP on F1. The synthesized ATP was bound to F1 to compose F1-ATP complex, suggesting that the energy input is necessary for the release of ATP from F1. The F1-ATP complex seemed to be formed also in the absence of DMSO. DMSO increased the affinity of F1 for Pi and shifted the equilibrium between F1-ATP complex and F1-ADP-Pi complex. Since it is the most simple ATP-synthesizing system ever known, it might be utilized to solve other problems about the mechanism of ATP synthesis. F1 has two types of nucleotide-binding sites, tight binding sites and exchangeble binding sites. However, it has not been exclusively determined which is the catalytic site of ATP synthesis. We found that ATP was formed by soluble F1 from ADP bound to the exchangeable binding site (s) but not from that bound to the tight binding sites.