L-1,2-propanediol exits more rapidly than L-lactaldehyde from Escherichia coli.

Catabolism of the six-carbon compound L-fucose results in formation of dihydroxyacetone phosphate (C-1-to-C-3 fragment) and L-lactaldehyde (C-4-to-C-6 fragment) as intermediates. The fate of lactaldehyde depends on the respiratory growth conditions. Aerobically, lactaldehyde is oxidized to L-lactate by an NAD-linked dehydrogenase (ald product). L-Lactate, in turn, is converted to pyruvate, which enters the general metabolic pool. Anaerobically, lactaldehyde is reduced to L-1,2-propanediol by an NADH-linked oxidoreductase (fucO product). L-1,2-Propanediol is excreted as a terminal fermentation product. In a previous study, we showed that retention of the C-4-to-C-6 fragment of fucose depended on the competition for lactaldehyde by aldehyde dehydrogenase and propanediol oxidoreductase (Y. Zhu and E.C.C. Lin, J. Bacteriol. 169:785-789, 1987). In this study, we compared the wild-type strain and isogenic mutant strains defective in ald, fucO, or both for ability to accumulate radioactivity when incubated with fucose labeled at either the C-1 or the C-6 position. The results showed that although blocking the oxidation of lactaldehyde prevented its assimilation, rapid exit of the 3-carbon unit occurred only when the compound was reduced to propanediol. Moreover, growth experiments on fucose indicated that a double ald fucO mutant accumulated inhibiting concentrations of lactaldehyde. The inner cell membrane therefore appears to be much more permeable to the 3-carbon alcohol than to the 3-carbon aldehyde. The almost instantaneous exit of propanediol appears to be a facilitated process.     

Cleft palate: more genetic lessons from mice

Cleft palate occurred in high frequency (14%) in the F2 generation of the cross between two stocks of mice, LGG and SELH, neither of which produces more than 2% cleft palate. The cleft palate trait results from a new combination of alleles that is not present in either parental stock. The lack of cleft palate in the F2 generation after outcrosses of both parental stocks to other strains shows that this new combination of alleles has specific contributions from both parental strains, and also that there must be at least two loci involved. A deficiency of Mod-1 homozygotes in the SELH/LGG F2 adults suggests that one of the loci involved may be linked to Mod-1 and that the number of loci involved is few. Significantly more F2 males (19%) than females (9%) were affected with cleft palate. The data can be explained by a two-locus epistatic model with a dominant mutation (P) at one locus that causes cleft palate when not suppressed by or compensated for by a dominant allele (S) at a second locus. The parental stocks would be PPSS and ppss. In the F2 generation, the new combinations PPss and Ppss would express cleft palate, a total expected of 19%. Similar new combinations of alleles at two loci may explain some instances of high occurrence of cleft palate or other developmental threshold traits in previously unaffected human families.     

Oxidative stress and condition-dependent sexual signals: more than just seeing red

The links between fitness, health, sexual signals and mate choice are complex and subject to ongoing study. In 1999, von Schantz et al. made the valuable suggestion that oxidative stress may be an important missing piece of this complex puzzle. Their suggestion has been enthusiastically tested, with over 300 studies citing their paper, but most effort has concerned carotenoid-based (and to a lesser extent melanin-based) visual signals, predominantly in birds and fishes. Today, we know a great deal more about oxidative stress and related physiology, in both a pathological and regulatory sense, than we did in 1999. We revisit von Schantz et al.'s predictions and, more importantly, highlight novel mechanisms that could link oxidative stress with a range of energetically demanding signals, greatly increasing the scope from visual signalling systems that are usually discussed and nearly always tested. In particular, we argue that differences between individuals in their ability to regulate physiology related to oxidative stress may be an important factor influencing the production of sexual signals and the costs that are incurred from investment.     

Rad17, the clamp loader that loads more than clamps

Rad17 is best known as a checkpoint clamp loader in the activation of ATR kinase signaling. A new study in The EMBO Journal suggests that it also plays a role in initial recruitment of the MRN complex to sites of DNA double‐strand breaks, thereby promoting early ATM checkpoint responses and homologous recombination repair.     

Recombinant thioredoxin peroxidase from Cryptosporidium parvum has more powerful antioxidant activity than that from Cryptosporidium muris

Cryptosporidium parvum can survive exposure to harsh environmental conditions, various disinfectants, and high doses of γ-irradiation. In an animal study, more than 25kGy of γ-irradiation was necessary to eliminate C. parvum infectivity from mice. In contrast, Cryptosporidium muris (murine Cryptosporidium), which lives in stomach epithelium, lost its infectivity in mice with 1kGy of γ-irradiation. Recently, it was found that thioredoxin peroxidase was highly expressed in C. parvum oocysts irradiated with high doses of γ-irradiation. Therefore we hypothesize that antioxidant activity of the thioredoxin peroxidase is involved in the radioresistance of C. parvum. To verify this, thioredoxin peroxidases of C. parvum (CpTPx) and C. muris (CmTPx) were expressed in Escherichia coli cells, and their antioxidant activities were compared. Both CpTPx and CmTPx belong to the 2-Cys family of peroxiredoxins. Hydrogen peroxide consumption was approximately 2- to 12-fold greater in recombinant CpTPx (rCpTPx) than in recombinant CmTPx (rCmTPx) in the presence of 0.2mM dithioerythritol or glutathione (GSH), respectively. The peroxidase activity of rCpTPx was highly enhanced by GSH, but that of rCmTPx was not. The minimum dose of rCpTPx required to protect supercoiled plasmid DNA from damage by metal-catalyzed oxidation was only 12% of that required with rCmTPx. The results showed that rCpTPx has more powerful antioxidant activity than rCmTPx. Further investigations on the role of CpTPx in the radioresistance of C. parvum are warranted.     

Kappa-chain constant-region gene sequences in genus Rattus: coding regions are diverging more rapidly than noncoding regions

We have determined the nucleotide sequence of a 1,200-base pair (bp) genomic fragment that includes the kappa-chain constant-region gene (C kappa) from two species of native Australian rodents, Rattus leucopus cooktownensis and Rattus colletti. Comparison of these sequences with each other and with other rodent C kappa genes shows three surprising features. First, the coding regions are diverging at a rate severalfold higher than that of the nearby noncoding regions. Second, replacement changes within the coding region are accumulating at a rate at least as great as that of silent changes. Third, most of the amino acid replacements are localized in one region of the C kappa domain--namely, the carboxy-terminal "bends" in the alpha-carbon backbone. These three features have previously been described from comparisons of the two allelic forms of C kappa genes in R. norvegicus. These data imply the existence of considerable evolutionary constraints on the noncoding regions (based on as yet undetermined functions) or powerful positive selection to diversify a portion of the constant-region domain (whose physiological significance is not known). These surprising features of C kappa evolution appear to be characteristic only of closely related C kappa genes, since comparison of rodent with human sequences shows the expected greater conservation of coding regions, as well as a predominance of silent nucleotide substitutions within the coding regions.      

In plants the alc gene expression system responds more rapidly following induction with acetaldehyde than with ethanol

It has recently been proposed that acetaldehyde is the physiological inducer of the alc gene system and hence indirectly the activator of the AlcA promoter in Aspergillus nidulans. Here we show that this chemical induces expression of a GUS (beta-D-glucuronidase) reporter under the control of the alc gene system in transgenic potato tubers more rapidly than ethanol allowing tighter control of transgene expression. Furthermore by analysis of metabolite levels we demonstrate that the application of inducer has few effects on metabolism. We propose that this system is therefore ideal for the temporal regulation of important metabolic enzyme activities.     

Building a cellular switch: more lessons from a good egg.

Xenopus oocytes mature in response to the steroid hormone progesterone. At the level of a population of oocytes, the response is graded-the higher the concentration of progesterone, the larger the fraction of oocytes that will mature-but at the level of the individual oocyte, the response is all-or-none. The all-or-none character of this cell fate switch is hypothesized to arise out of two properties of the signal transduction machinery that mediates maturation, positive feedback, and ultrasensitivity. This combination of positive feedback plus ultrasensitivity crops up again and again in cellular switches, from the lysis-lysogeny switch in phage-infected bacteria to the action potential in neurons.     

Nucleocytoplasmic transport: more than the usual suspects

A paper in the August 9 issue of Cell describes a novel role for the nucleoporin Npap60/Nup50 as a soluble cofactor in importin-alpha:beta-mediated nuclear protein import. These findings add a new dimension of complexity to the current understanding of protein transport pathways.     

Visual perception: more than meets the eye

A recent study shows that objects changing in colour, luminance, size or shape appear to stop changing when they move. These and other compelling illusions provide tantalizing clues about the mechanisms and limitations of object analysis.     

Bacterial phytochromes: more than meets the light

Phytochromes are environmental sensors, historically thought of as red/far-red photoreceptors in plants. Their photoperception occurs through a covalently linked tetrapyrrole chromophore, which undergoes a light-dependent conformational change propagated through the protein to a variable output domain. The phytochrome composition is modular, typically consisting of a PAS-GAF-PHY architecture for the N-terminal photosensory core. A collection of three-dimensional structures has uncovered key features, including an unusual figure-of-eight knot, an extension reaching from the PHY domain to the chromophore-binding GAF domain, and a centrally located, long α-helix hypothesized to be crucial for intramolecular signaling. Continuing identification of phytochromes in microbial systems has expanded the assigned sensory abilities of this family out of the red and into the yellow, green, blue, and violet portions of the spectrum. Furthermore, phytochromes acting not as photoreceptors but as redox sensors have been recognized. In addition, architectures other than PAS-GAF-PHY are known, thus revealing phytochromes to be a varied group of sensory receptors evolved to utilize their modular design to perceive a signal and respond accordingly. This review focuses on the structures of bacterial phytochromes and implications for signal transmission. We also discuss the small but growing set of bacterial phytochromes for which a physiological function has been ascertained.     

Src: more than the sum of its parts

The Src family of protooncoproteins is required for prc through at least two phases of the cell cycle and for sc cell-type-specific functions. Recent crystal structures of fragments of two representatives reveal a compact am their Src-homology 3 (SH3), SH2 and catalytic domai embodies an unexpected mechanism of regulation. Th. the enzymatic activity of Src is controlled by intramol associations between the SH2 domain and C-tail and SH3 domain and a surprising internal target. The stn highlight a mechanism by which substrates can comp internal sequences for binding to the SH3 and SH2 do thereby stimulating kinase activity. This implies that distinction between upstream activators and downstre will sometimes be ambiguous.     

ATR signalling: more than meeting at the fork

Preservation of genome integrity via the DNA-damage response is critical to prevent disease. ATR (ataxia telangiectasia mutated- and Rad3-related) is essential for life and functions as a master regulator of the DNA-damage response, especially during DNA replication. ATR controls and co-ordinates DNA replication origin firing, replication fork stability, cell cycle checkpoints and DNA repair. Since its identification 15 years ago, a model of ATR activation and signalling has emerged that involves localization to sites of DNA damage and activation through protein-protein interactions. Recent research has added an increasingly detailed understanding of the canonical ATR pathway, and an appreciation that the canonical model does not fully capture the complexity of ATR regulation. In the present article, we review the ATR signalling process, focusing on mechanistic findings garnered from the identification of new ATR-interacting proteins and substrates. We discuss how to incorporate these new insights into a model of ATR regulation and point out the significant gaps in our understanding of this essential genome-maintenance pathway.