This review has summarized the recent knowledge about structure and expression of the 5S RNA genes in eucalyotes. 5S RNA is an integral component of the large subunit of the ribosome. Eucaryotic cells contain per haploid 10102-104 copies of the gene for this small RNA. Three different types of genomic organization of the 5S RNA genes have been observed by gene cloning and hybridization experiments. Primary structure of the various 5S RNA genes and their adjacent regions has been determined, which pnovides lots of clues to understand function and evolution of the gene. In parallel with the structural analyses, mechanisms of 5S RNA transcription and its regulation have been studied by using in vitro transcription systems, The central region of the 5S RNA gene directs the accurate and effective initiation of transcription. Besides RNA polymerase III, at least two protein factors concern the initiation process, and one of these proteins named "TFIIIA" binds specifically to the central region of the gene.
The polyphosphoinositides (PPI), diphosphoinositide (DPI) and triphosphoinositide (TPI), are quantitatively minor components of various eukaryotic tissues. They are of special biochemical interst because of their extraordinarily high affinity for Ca++ and the rapid turnover rate of their monoesterified phosphate groups. In addition, their metabolic turnover is accelerated with appropriate stimulations to the tissues. In this paper, we reviewed representative studies os PPI and discussed possible roles of these lipids in the stimulus-response coupling via biological membranes. Several results from the studies on the PPI in nervous tissues suggested a strong correlation between PPI and the, K+ -permeability of excitable membranes. It was also mentioned that PPI might play essential roles in ACTH-activated memory in the limbic system, ACTH-activated steroidgenesis in the adrenal cortex, ····etc. In contrast with the phosphatidylinositol (PI)-breakdown or the phospholipid-methylation that is suggested to contribute to an earlier step in the information transduction process via cellular membranes, PPI-metabolism may have important roles at a later step in the transduction process and contribute to various cellular responses.
An early stage of developing eggs of the seaweed Fucus is summarized briefly. A Fucus egg, which is initially apolar, forms a rhizoid 12-14 hours after fertilization. After a latent period when no egg responds to external vectors, the polarity becomes sensitive to some external vectors as a unilateral light. Fucus eggs also tend to form rhizoids to each other when eggs are sufficiently close. As a probable mechanism to explain the rhizoid formation, an electrophoresis mechanism is mentioned in detail. Based on a bridging activity of Ca2+ in the membrane formation, an appearance of sensitivity in Fucus eggs is understood as a kind of bifurction phenomenon far from equilibrium. The appearance of polarity influenced by neighboring eggs is also discussed in terms of a self-organization process through the increasing number of components.
The current idea that the sliding between thick and thin filaments in muscle is caused by the "rotation" of the myosin cross-bridge head is critically reviewed, together with the hystorical backgrounds having led to the development of the head rotation model of muscle contraction. The head rotation model involves two basic assumptions, i.e. (i) the rotation of the cross-bridge head and (ii) the presence of the elastic component in the SII portion of the cross-bridge. In the review, emphasis is particularly placed on the examination about the two basic assumptions. Our recent study with glycerinated rabbit psoas fibers showed that most of the muscle compliance in rigor fibers originates in the cross-bridges and another our study on the effect of the chemical cross-linking of the SII portion onto the surface of the thick filaments in rigor fibers on the muscle stiffness showed that the SII portion is not compliant. Since there has been no evidence for the head rotation in rigor fibers when the fibers are stretched as well as in active muscle fibers, we suppose that the proximal domain of the cross-bridge head (SI portion) is compliant. The implication of the compliance in the proximal domain of the cross-bridge head in the mechanisms of the muscle force generation is discussed.