Dark incubation causes reinitiation of cell cycle events in Anacystis nidulans.

Synchronized cultures of Anacystis nidulans (Synechococcus PCC 6301), an obligate phototroph, are obtained by incubating exponential cultures in the dark for 12 to 16 h. A temporal and sequential order of macromolecular synthesis is observed within the cell division cycle of a synchronously dividing culture in the light. Apparently, dark incubation causes the cells to realign their cellular activities in such a way that all cells emerge from the dark and grow synchronously in the light. A study was conducted to explore the possible mechanisms responsible for the putative dark-induction process. Samples were taken at various times from a synchronized culture and were subjected to another round of dark incubation for 16 h. When these cultures were returned to the light, the cell number increased from 3 h and doubled at about 7 h. The protein, RNA, and DNA contents started to increase in order well before 3 h. This general pattern of cellular activities, observed for nearly all samples (i.e., for cells of different physiological ages), indicated that the dark incubation period caused the ongoing cell cycle to abort and a new cell cycle to be reinitiated under light growth conditions.     

Control models of cell cycle transit, exit, and arrest

Several distinct cycles mediate the events which occur between one cell division and the next. In micro-organisms there are generally two cycles. One governs biomass growth, the other DNA synthesis and cell division. In higher eukaryotes there can be as many as four distinct cycles, with growth, DNA synthesis, cell division, and nuclear division each possessing its own functional sequence of events. These cycles are controlled and coordinated by several different regulatory mechanisms. Restriction points are specific steps in the cycle whose completion is governed by external regulatory agents. One set of restriction points requires nutrients and growth hormones for step completion. Another set serves as receptors for differentiating factors which cause cycle arrest and initiate cellular differentiation. There is currently a debate as to whether restriction point inhibition involves permanent arrest or temporary arrest with a stochastic arrested-state residence time controlled by a transition probability mechanism. Tissue sizing is a process of negative feedback inhibition mediated by intercellular communication via cell surface contact and the extracellular matrix. Sizers commonly operate throughout broad portions of the cycle and appear to cause a slowing of cycle transit velocity rather than arrest. Sizers are probably the major regulatory mechanism for cell growth under conditions of nutrient and growth factor excess. They also generate compensatory proliferation following wounding or cell death. A growing body of evidence suggests that both the transit velocity, with which cells move through their several cycles, and the coordination of the cycles are controlled by intracellular regulatory mechanisms which behave as biological oscillators. These oscillators trigger complex sequences of events such as DNA synthesis and cell division.     

Chloroplast Protein Synthesis in the Chromophytic Alga Olisthodiscus luteus: Cell Cycle Analysis

This study represents the first report on chloroplast protein synthesis during the synchronous cell growth of a chromophytic (chlorophyll a,c) plant. When the unicellular alga Olisthodiscus luteus is maintained on a 12-hour light:12-hour dark cycle, cell and chloroplast number double every 24 hours. A temporal separation between these two events occurs. Measurements of chloroplast and total cellular protein values suggest that polypeptide synthesis occurs mainly in the light portion of the cell cycle, and pulse chase studies demonstrate that chloroplast proteins made in the light are not degraded in the dark. Data support the following conclusions: (a) a similar complement of chloroplast DNA coded proteins is made at all phases of the light portion of the cell cycle, and (b) chloroplast protein synthesis is a light rather than a cell cycle mediated response.     

Cell proliferation and growth in C. elegans.

The cell division and differentiation events that occur during the development of the nematode Caenorhabditis elegans are nearly identical between different individuals, a feature that distinguishes this organism from larger and more complex metazoans, such as humans and Drosophila. In view of this discrepancy, it might be expected that the regulation of cell growth, division and differentiation in C. elegans would involve mechanisms separate from those utilized in larger animals. However, the results of recent genetic, molecular and cellular studies indicate that C. elegans employs an arsenal of developmental regulatory mechanisms quite similar to those wielded by its arthropod and vertebrate relatives. Thus, the nematode system is providing both novel and complementary insights into the general problem of how growth and patterning events are integrated in development. This review offers a general perspective on the regulation of cell division and growth in C. elegans, emphasizing recent studies of these crucial aspects of development.     

Cell Cycle Regulation in Marine Synechococcus sp. Strains

The cell cycle behavior of four marine strains of the unicellular cyanobacterium Synechococcus sp. was analyzed by examining the DNA frequency distributions of exponentially growing and dark-blocked populations and by considering the patterns of change in these distributions during growth under a diel light-dark cycle. The two modes of cell cycle regulation previously identified in a freshwater and coastal marine Synechococcus isolate, respectively, were represented among the three open-ocean strains we examined. The first of these modes of regulation is consistent with the slow-growth case of the widely accepted prokaryotic cell cycle paradigm. The second appears to involve asynchronous initiation of chromosome replication, the presence of multiple chromosome copies at low growth rates, and variability in chromosome copy number among cells in the population. These characteristics suggest the involvement of a large probabilistic component in cell cycle regulation which could make the application of cell cycle-based estimators of in situ growth rate to Synechococcus populations problematic.     

Light regulation of the cell cycle in Euglena gracilis bacillaris

We have studied the light regulation of the cell division cycle in the photosynthetic alga Euglena gracilis bacillaris. Euglena grown under phototrophic conditions are easily synchronized to a 12 h light-12 h dark regime. By inoculating stationary phase, nondividing cells into fresh media and exposing the diluted cells to either light or darkness, we have determined that initiation of DNA synthesis for the cell division cycle is light dependent. By varying the length of time in light to which synchronized cells are exposed, we have shown that commitment to the cell cycle requires exposure to more than 6 h of light. We propose that this is to allow the accumulation, through photosynthetic electron transport, of an initiating factor that will enable DNA synthesis to begin. Flow cytometry analysis also shows that once cells are committed to the cell cycle, they complete the cycle in the dark, so mitosis is a light-independent step.     

Sphingosine-1-Phosphate Metabolism and Intestinal Tumorigenesis: Lipid Signaling Strikes Again

Superficially similar traits in phylogenetically unrelated species often result from adaptation to common selection pressures. Examples of convergent evolution are known at the levels of whole organisms, organ systems, gene networks and specific proteins. The phenotypic properties of living things, on the other hand, are determined in large part by complex networks of interacting proteins. Here we present a mathematical model of the network of proteins that controls DNA synthesis and cell division in the alpha-proteobacterium, Caulobacter crescentus. By comparing the protein regulatory circuits for cell reproduction in Caulobacter with that in budding yeast (Saccharomyces cerevisiae), we suggest that convergent evolution may have created similar molecular reaction networks in order to accomplish the same purpose of coordinating DNA synthesis to cell division. Although the genes and proteins involved in cell cycle regulation in prokaryotes and eukaryotes are very different and (apparently) phylogenetically unrelated, they seem to be wired together in similar regulatory networks, which coordinate cell cycle events by identical dynamical principles.     

Circadian variability in the photobiology of Phaeodactylum tricornutum: pigment content

Circadian variations of pigment content in the diatom Phaeodactylumtricornutum were analyzed in different light regimes. The studywas aimed at discerning the role of putative endogenous controlsfrom the constraint imposed by the alternation of light (L)and dark (D) periods. Our experiments showed that in a typicalLD cycle of illumination, pigment synthesis follows the somaticgrowth of the cell, both arresting during D periods. In particular,the diurnal increase of chlorophyll a content was proportionalto the increase in cell size and preceding cell division, occurringat night. By contrast, diadinoxanthin and ß–carotene displayed different phases, which is likely to be relatedto their involvement in photoprotection mechanisms. The experimentsalso showed that the synthesis of both photosynthetic and photoprotectivepigments was dependent not only on light availability and thephasing of somatic growth, but also responded to other internalregulation. Over the time scale of the experiments (hours todays), the removal of LD–DL triggers impaired cell physiology,whereas the circadian patterns in pigment synthesis persisted.Our results support the hypothesis that an internal regulationof cell biosynthetic machinery can improve phytoplankton fitness,even in high variable environments such as the oceanic mixedlayers. Therefore, we suggest that phytoplankton growth dependsnot only on the availability of external resources, but alsoon internal regulatory mechanisms whose unveiling would furtherour understanding of phytoplankton diversity and dynamics.     

Effect of lectins on auxin-induced elongation and wall loosening in oat coleoptile and azuki bean epicotyl segments

A marine unicellular aerobic nitrogen-fixing cyanobacterium Synechococcus sp. strain Miarni BG 043511 was pretreated with different light and dark regimes in order to induce higher growth synchrony. A pretreatment of two dark and light cycles of 16 h each yielded good synchrony for 3 cell division cycles. Longer dark treatments decreased the degree of synchrony and shorter dark treatments caused irregular cell division. Once synchronous culture was established, distinct phases of cellular carbohydrate accumulation and cellular carbohydrate degradation were observed even under continuous illumination. Changes in carbohydrate content were repeated in a cyclic manner with approximately 20 h intervals, the same as the cell division cycle. This change in carbohydrate metabolism provided a good index of growth synchrony under nitrogen-fixing conditions.
Photosynthetic oxygen evolution and nitrogen fixation capabilities and their activities in near, in situ, culture conditions were measured in well synchronized cultures of this strain under continuous illumination. Distinct oscillations of both photosynthetic oxygen evolution and nitrogen fixation capabilities with ca 20-h intervals, similar to the interval of the cell division cycle, were observed for three cycles. However, the activities of photosynthetic oxygen evolution were inversely correlated with those of nitrogen fixation. During the nitrogen fixation period, net oxygen consumption was observed even in the light under conditions approximating in situ culture conditions. The phase of temporal appearance of nitrogenase activity during the cell division cycle coincided with the phase of carbohydrate net degradation. These data indicate that this unicellular cyanobacterium can grow diazotrophically under conditions of continuous illumination by the segregation of photosynthesis and nitrogen fixation within a cell division cycle.     

Insights into regulation of the mammalian cell cycle from studies on spermatogenesis using genetic approaches in animal models

The genetic hierarchy controlling mitosis and especially meiosis during gamete formation is not well understood, even in less complicated systems such as the yeasts. Meiotic divisions are obviously restricted to germ line cells and as such likely require mechanisms of cell cycle control that do not function and may not exist in somatic cells. While male and female germ cells have stages of cell cycle regulation in common, the timing of these events and the stage of development at which these events occur differ in the two sexes. Understanding the genetic program controlling the mitotic and meiotic divisions of the germ line represents a unique opportunity for providing insight into cell cycle control in vivo. Elucidating the key control points and proteins may also enhance our understanding of the etiology of infertility and provide new directions for contraception.     

Mitochondrial involvement in the point of no return in neuronal apoptosis

Programmed cell death (PCD) contributes to development, maintenance, and pathology in various tissues, including the nervous system. Many molecular, biochemical, and genetic events occur within cells undergoing PCD. Some of these events are incompatible with long-term cell survival because they have irreversible, catastrophic consequences. The onset of such changes marks the point of no return, a decisive regulatory event termed 'the commitment-to-die.' In this review, we discuss events that underlie the commitment-to-die in nerve growth factor-deprivation-induced death of sympathetic neurons. Findings in this model system implicate the mitochondrion as an important site of regulation for the commitment-to-die in the presence or absence of caspase inhibition.     

Developmental regulation of choline sulfatase and aryl sulfatase in Neurospora crassa

Regulation of the synthesis of several enzymes of sulfur metabolism in Neurospora is a function of both metabolic regulation and the genetic control exerted by the cys-3 and scon regulatory genes. Additional control mechanisms appear to regulate the synthesis of choline sulfatase and aryl sulfatase in different developmental stages of the life cycle. The metabolic regulation of enzyme synthesis in conidia differs from that which occurs in the mycelial stage. During conidial germination and mycelial outgrowth, the synthesis of these enzymes is not coordinate but begins at different times and occurs at different rates. A rapid and early synthesis of choline sulfatase was observed during conidial germination under derepressing conditions; furthermore, synthesis of the enzyme also occurred for a brief period in germinating conidia even in the presence of repressing levels of sulfate. The results of this study suggest that several enzymes of sulfur metabolism are independently controlled by a developmental system which is superimposed upon the cys-3 regulatory mechanism. It was also found that choline sulfatase undergoes rapid turnover while aryl sulfatase is a stable species.