Structural evidence that the P/Q domain of ZipA is an unstructured,flexible tether between the membrane and the C-terminal FtsZ-binding domain

The cell division protein ZipA has an N-terminal transmembrane domain and a C-terminal globular domain that binds FtsZ. Between them are a charged domain and a P/Q domain rich in proline and glutamine that has been proposed to be an unfolded polypeptide. Here we provide evidence obtained by electron microscopy that the P/Q domain is a flexible tether ranging in length from 8 to 20 nm and invisible in rotary shadowing electron microscopy. We estimated a persistence length of 0.66 nm, which is similar to the persistence lengths of other unfolded and unstructured polypeptides.     

A general modeling framework for describing spatially structured population dynamics

Variation in movement across time and space fundamentally shapes the abundance and distribution of populations. Although a variety of approaches model structured population dynamics, they are limited to specific types of spatially structured populations and lack a unifying framework. Here, we propose a unified network‐based framework sufficiently novel in its flexibility to capture a wide variety of spatiotemporal processes including metapopulations and a range of migratory patterns. It can accommodate different kinds of age structures, forms of population growth, dispersal, nomadism and migration, and alternative life‐history strategies. Our objective was to link three general elements common to all spatially structured populations (space, time and movement) under a single mathematical framework. To do this, we adopt a network modeling approach. The spatial structure of a population is represented by a weighted and directed network. Each node and each edge has a set of attributes which vary through time. The dynamics of our network‐based population is modeled with discrete time steps. Using both theoretical and real‐world examples, we show how common elements recur across species with disparate movement strategies and how they can be combined under a unified mathematical framework. We illustrate how metapopulations, various migratory patterns, and nomadism can be represented with this modeling approach. We also apply our network‐based framework to four organisms spanning a wide range of life histories, movement patterns, and carrying capacities. General computer code to implement our framework is provided, which can be applied to almost any spatially structured population. This framework contributes to our theoretical understanding of population dynamics and has practical management applications, including understanding the impact of perturbations on population size, distribution, and movement patterns. By working within a common framework, there is less chance that comparative analyses are colored by model details rather than general principles.     

Learning from regeneration research organisms: The circuitous road to scar free wound healing

The skin is the largest organ in the body and plays multiple essential roles ranging from regulating temperature, preventing infection and ultimately defining who we are physically. It is a highly dynamic organ that constantly replaces the outermost cells throughout life. However, when faced with a major injury, human skin cannot restore a significant lesion to its original functionality, instead a reparative scar is formed. In contrast to this, many other species have the unique ability to regenerate full thickness skin without formation of scar tissue. Here we review recent advances in the field that shed light on how the skin cells in regenerative species react to injury to prevent scar formation versus scar forming humans.     

A molecular basis for discrete size variation in human ribosomal DNA.

The tandemly repeated human ribosomal RNA (rRNA) genes contain a region of size heterogeneity that is present in the nontranscribed spacer of every individual examined. This heterogeneity has been previously examined by Southern analysis of BamHI-digested human DNA. Using a ribosomal DNA (rDNA) probe specific for the 3' end of the 28S rRNA gene, at least four discrete sizes of BamHI fragments were seen in human populations. Molecular analysis of the cloned DNA from this region reveals tandem duplication of a segment of spacer rDNA located 388 base pairs (bp) 3' to the end of the 28S ribosomal RNA gene. Five hundred fifty bp of DNA, flanked on either side by a 150-bp repeated element, is either duplicated or deleted to produce a series of spacers that differ in size by 850 bp. These duplications/deletions appear to be the product of unequal homologous exchange, mediated by the small repeated element. Thus, human rDNA fragments cloned in lambda vectors and propagated in E. coli generate the same apparent size variation seen in genomic DNA. This study suggests that unequal homologous exchange is the molecular basis for the observed length heterogeneity in the spacer rDNA and may be a common mechanism for the generation of human genetic diversity.     

Interrelationships of protein and DNA syntheses during replication of mammalian cells.

During the replication of chromatin, the syntheses of the histone protein and DNA components are closely coordinated but not totally linked. The interrelationships of total protein synthesis, histone protein synthesis, DNA synthesis, and mRNA levels have been investigated in Chinese hamster ovary cells subjected to several different types of inhibitors in several different temporal combinations. The results from these studies and results reported elsewhere can be brought together into a consistent framework which combines the idea of autoregulation of histone biosynthesis as originally proposed by W. B. Butler and G. C. Mueller (Biochim. Biophys. Acta 294:481-496, 1973] with the presence of basal histone synthesis and the effects of protein synthesis on DNA synthesis. The proposed framework obviates the difficulties of Butler and Mueller's model and may have wider application in understanding the control of cell growth.     

GTP hydrolysis during microtubule assembly

The GTP cap model of dynamic instability [Mitchison, T., & Kirschner, M.W. (1984) Nature (London) 312, 237] postulates that a GTP cap at the end of most microtubules stabilizes the polymer and allows continuing assembly of GTP-tubulin subunits while microtubules without a cap rapidly disassemble. This attractive explanation for observed microtubule behavior is based on the suggestion that hydrolysis of GTP is not coupled to assembly but rather takes place as a first-order reaction after a subunit is assembled onto a polymer end. Carlier and Pantaloni [Carlier, M., & Pantaloni, D. (1981) Biochemistry 20, 1918] reported a lag of hydrolysis behind microtubule assembly and a first-order rate constant for hydrolysis (kh) of 0.25/min. A lag has not been demonstrated by other investigators, and a kh value that specifies such a slow rate of hydrolysis is difficult to reconcile with reported steady-state microtubule growth rates and frequencies of disassembly. We have looked for a lag using tubulin free of microtubule-associated protein at concentrations of 18.5-74 microM, assembly with and without glycerol, and two independent assays of GTP hydrolysis. No lag was observed under any of the conditions employed, with initial rates of hydrolysis increasing in proportion to rates of assembly. If hydrolysis is uncoupled from assembly, we estimate that kh must be at least 2.5/min and could be much greater, a result that we argue may be advantageous to the GTP cap model. We also describe a preliminary model of assembly coupled to hydrolysis that specifies formation and loss of a GTP cap, thus allowing dynamic instability.     

Theoretical and experimental yields for photoautotrophic, mixotrophic, and photoheterotrophic growth

Available electron methods are presented and used to estimate theoretical energetic growth yields for photoautotrophic, mixotrophic, and photoheterotrophic growth of algae and photosynthetic bacteria. The theoretical yields are compared to experimental values reported previously. For photoautotrophic and mixotrophic growth of algae experimental values that approach and even exceed the theoretical values have been reported in the literature. For photosynthetic bacteria experimental yields are much smaller than thetheoretical maximum values.     

Contraceptive practices and fertility among Southeast Asian, black, and white mothers attending a maternal infant care program

The purpose of this analysis was to determine if there were differences in selected fertility characteristics including parity, pregnancy spacing, age at 1st pregnancy, age of menarche, breastfeeding postpartum, and contraceptive practices among white, black, Hmong, and other Southeast Asian mothers attending a maternal infant care program in Minneapolis, Minnesota, during 1980-1982. White and black mothers were younger than the Hmong and other Asian mothers. The lowest mean age of 1st pregnancy was among blacks. Ages of 1st pregnancy were similar for whites, Hmong, and other Asians, although the mean age of menarche was approximately 2 years later for Hmong and other Southeast Asian mothers compared to the white and black mothers. Based on self reports at the 1st postpartum visit 1 month after delivery, 39% of the whites and 25% of the blacks were breastfeeding. In comparison 8.7% of the Hmong and 17% of the other southeast Asian mothers were breastfeeding. Maternal aged age at 1st pregnancy were significant predictors of parity for whites, blacks, Hmong, and other Southeast Asians. Ever-use of contraception was significant predictor of parity only for Hmong. The highest proportion of ever-users of contraception was among the white mothers (80%) followed by the blacks (69.3%) and other Asian mothers (34.85). Hmong mothers had the lowest proportion having used contraception (17.1%).