Mutational and comparative analysis of streptolysin O, an oxygen-labile streptococcal hemolysin

The structural gene of streptolysin O was cloned from Streptococcus pyogenes strain Sa and S. equisimilis H46A, and the nucleotide sequences were compared with those of strain Richards. To obtain the minimal active fragment of the toxin and to elucidate structure-function relationships in hemolytic function, streptolysin O mutants deleted in N- and C-terminal regions were constructed. Internal amino acid residues were also replaced by introduction of point mutations. Analyses of these mutants showed that considerable activity was retained even after deletion of the N-terminal 107 residues, but genetic removal of the ultimate C-terminal residue resulted in a marked decrease in hemolytic function. By removal in succession, hemolytic activity declined exponentially, and only 0.002% of the activity remained after deletion of the C-terminal four residues. Nucleotide replacement experiments indicated pivotal roles of I202, V217, D324-L325, V339, and H469 residues in hemolysis.     

Morphological development of the mouse gonad in tda-1 XY sex reversal

tda-1 XY sex reversal occurs when the Y chromosome of at least some populations of wild Mus musculus domesticus is placed on the C57BL/6J genomic background. Gross anatomical observations have previously revealed morphological similarities among fetal ovotestes of tda-1 and Tas-inherited XY sex reversals and BALB/cWt mosaic hermaphrodites. We studied the histology of tda-1 XY sex-reversed gonads, ranging in age from day 14 of gestation to adult. The obtained data revealed additional similarities with ovotestes of BALB/cWt mosaic hermaphrodites as well as with ovotestes of hermaphrodites found in XXSxr and XX/XY chimeras. It is proposed that ovotestes occurring in these various hermaphroditic conditions may be formed through a common pathway.     

Wt1 negatively regulates beta-catenin signaling during testis development

beta-Catenin, as an important effector of the canonical Wnt signaling pathway and as a regulator of cell adhesion, has been demonstrated to be involved in multiple developmental processes and tumorigenesis. beta-Catenin expression was found mainly on the Sertoli cell membrane starting from embryonic day 15.5 in the developing testes. However, its potential role in Sertoli cells during testis formation has not been examined. To determine the function of beta-catenin in Sertoli cells during testis formation, we either deleted beta-catenin or expressed a constitutively active form of beta-catenin in Sertoli cells. We found that deletion caused no detectable abnormalities. However, stabilization caused severe phenotypes, including testicular cord disruption, germ cell depletion and inhibition of Müllerian duct regression. beta-Catenin stabilization caused changes in Sertoli cell identity and misregulation of inter-Sertoli cell contacts. As Wt1 conditional knockout in Sertoli cells causes similar phenotypes to our stabilized beta-catenin mutants, we then investigated the relationship of Wt1 and beta-catenin in Sertoli cells and found Wt1 inhibits beta-catenin signaling in these cells during testis development. Wt1 deletion resulted in upregulation of beta-catenin expression in Sertoli cells both in vitro and in vivo. Our study indicates that Sertoli cell expression of beta-catenin is dispensable for testis development. However, the suppression of beta-catenin signaling in these cells is essential for proper testis formation and Wt1 is a negative regulator of beta-catenin signaling during this developmental process.     

A novel bifunctional aspartate kinase-homoserine dehydrogenase from the hyperthermophilic bacterium,Thermotoga maritima

ABSTRACT

The orientation of the three domains in the bifunctional aspartate kinase-homoserine dehydrogenase (AK-HseDH) homologue found in Thermotoga maritima totally differs from those observed in previously known AK-HseDHs; the domains line up in the order HseDH, AK, and regulatory domain. In the present study, the enzyme produced in Escherichia coli was characterized. The enzyme exhibited substantial activities of both AK and HseDH. L-Threonine inhibits AK activity in a cooperative manner, similar to that of Arabidopsis thaliana AK-HseDH. However, the concentration required to inhibit the activity was much lower (K_0.5 = 37 μM) than that needed to inhibit the A. thaliana enzyme (K_0.5 = 500 μM). In contrast to A. thaliana AK-HseDH, Hse oxidation of the T. maritima enzyme was almost impervious to inhibition by L-threonine. Amino acid sequence comparison indicates that the distinctive sequence of the regulatory domain in T. maritima AK-HseDH is likely responsible for the unique sensitivity to L-threonine.

Abbreviations: AK: aspartate kinase; HseDH: homoserine dehydrogenase; AK–HseDH: bifunctional aspartate kinase–homoserine dehydrogenase; AsaDH: aspartate–β–semialdehyde dehydrogenase; ACT: aspartate kinases (A), chorismate mutases (C), and prephenate dehydrogenases (TyrA, T).     

Cooperative Degradation of Chitin by Extracellular and Cell Surface-Expressed Chitinases from Paenibacillus sp. Strain FPU-7

Chitin, a major component of fungal cell walls and invertebrate cuticles, is an exceedingly abundant polysaccharide, ranking next to cellulose. Industrial demand for chitin and its degradation products as raw materials for fine chemical products is increasing. A bacterium with high chitin-decomposing activity, Paenibacillus sp. strain FPU-7, was isolated from soil by using a screening medium containing α-chitin powder. Although FPU-7 secreted several extracellular chitinases and thoroughly digested the powder, the extracellular fluid alone broke them down incompletely. Based on expression cloning and phylogenetic analysis, at least seven family 18 chitinase genes were found in the FPU-7 genome. Interestingly, the product of only one gene (chiW) was identified as possessing three S-layer homology (SLH) domains and two glycosyl hydrolase family 18 catalytic domains. Since SLH domains are known to function as anchors to the Gram-positive bacterial cell surface, ChiW was suggested to be a novel multimodular surface-expressed enzyme and to play an important role in the complete degradation of chitin. Indeed, the ChiW protein was localized on the cell surface. Each of the seven chitinase genes (chiA to chiF and chiW) was cloned and expressed in Escherichia coli cells for biochemical characterization of their products. In particular, ChiE and ChiW showed high activity for insoluble chitin. The high chitinolytic activity of strain FPU-7 and the chitinases may be useful for environmentally friendly processing of chitin in the manufacture of food and/or medicine.     

Competence-dependent endogenous DNA rearrangement and uptake of extracellular DNA give a natural variant of Streptococcus mutans without biofilm formation

The production of water-insoluble glucan (WIG) enables Streptococcus mutans to survive and persist in the oral niche. WIG is produced from sucrose by glucosyltransferase encoded tandemly by the highly homologous gtfB and gtfC genes. Conversely, a single hybrid gene from the endogenous recombination of gtfB and gtfC is easily generated using RecA, resulting in S. mutans UA159 WIG- (rate of ～1.0×10(-3)). The pneumococcus recA gene is regulated as a late competence gene. comX gene mutations did not lead to the appearance of WIG- cells. The biofilm collected from the flow cell had more WIG- cells than among the planktonic cells. Among the planktonic cells, WIG- cells appeared after 16 h and increased ～10-fold after 32 h of cultivation, suggesting an increase in planktonic WIG- cells after longer culture. The strain may be derived from the biofilm environment. In coculture with donor WIG+ and recipient WIG- cells, the recipient cells reverted to WIG+ and acquired an intact gtfBC region from the environment, indicating that the uptake of extracellular DNA resulted in the phenotypic change. Here we demonstrate that endogenous DNA rearrangement and uptake of extracellular DNA generate WIG- cells and that both are induced by the same signal transducer, the com system. Our findings may help in understanding how S. mutans can adapt to the oral environment and may explain the evolution of S. mutans.