Cellulolysis is severely affected in Clostridium cellulolyticum strain cipCMut1

Progress towards understanding the molecular basis of cellulolysis by Clostridium cellulolyticm was obtained through the study of the first cellulolysis defective mutant strain, namely cipCMut1. In this mutant, a 2 659 bp insertion element, disrupts the cipC gene at the sequence encoding the seventh cohesin of the scaffoldin CipC. cipC is the first gene in a large 'cel' gene cluster, encoding several enzymatic subunits of the cellulosomes, including the processive cellulase Cel48F, which is the major component. Physiological and biochemical studies showed that the mutant strain was affected in cellulosome synthesis and severely impaired in its ability to degrade crystalline cellulose. It produced small amounts of a truncated CipC protein (P120), which had functional cohesin domains and assembled complexes which did not contain any of the enzymes encoded by genes of the 'cel' cluster. The mutant cellulolytic system was mainly composed of three proteins designated P98, P105 and P125. Their N-termini did not match any of the known cellulase sequences from C. cellulolyticum. A large amount of entire CipC produced in the cipCMut1 strain by trans-complementation with plasmid pSOScipC did not restore the cellulolytic phenotype, in spite of the assembly of a larger amount of complexes. The complexes produced in the mutant and complemented strains contained at least 12 different dockerin-containing proteins encoded by genes located outside of the 'cel' cluster. The disturbances observed in the mutant and trans-complemented strains were the result of a strong polar effect resulting from the cipC gene disruption. In conclusion, this study provided genetic evidence that the cellulases encoded by the genes located in the 'cel' cluster are essential for the building of cellulosomes efficient in crystalline cellulose degradation.     

Use of antisense RNA to modify the composition of cellulosomes produced by Clostridium cellulolyticum

The enzymatic composition of the cellulosomes produced by Clostridium cellulolyticum was modified by inhibiting the synthesis of Cel48F that is the major cellulase of the cellulosomes. The strain ATCC 35319 (pSOSasrF) was developed to over-produce a 469 nucleotide-long antisense-RNA (asRNA) directed against the ribosome-binding site region and the beginning of the coding region of the cel48F mRNAs. The cellulolytic system secreted by the asRNA-producing strain showed a markedly lower amount of Cel48F, compared to the control strain transformed with the empty plasmid (pSOSzero). This was correlated with a 30% decrease of the specific activity of the cellulolytic system on Avicel cellulose, indicating that Cel48F plays an important role in the recalcitrant cellulose degradation. However, only minor effects were observed on the growth parameters on cellulose. In both transformant strains, cellulosome production was found to be reduced and two unknown proteins (P105 and P98) appeared as major components of their cellulolytic systems. These proteins did not contain any dockerin domain and were shown to be not included into the cellulosomes; they are expected to participate to the non-cellulosomal cellulolytic system of C. cellulolyticum.     

Epimerization of chenodeoxycholic acid to ursodeoxycholic acid by Clostridium baratii isolated from human feces

Several H2-producing fermentative anaerobic bacteria including Clostridium, Klebsiella and Fusobacteria degraded octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) (36 microM) to formaldehyde (HCHO) and nitrous oxide (N2O) with rates ranging from 5 to 190 nmol h(-1)g [dry weight] of cells(-1). Among these strains, C. bifermentans strain HAW-1 grew and transformed HMX rapidly with the detection of the two key intermediates the mononitroso product and methylenedinitramine. Its cellular extract alone did not seem to degrade HMX appreciably, but degraded much faster in the presence of H2, NADH or NADPH. The disappearance of HMX was concurrent with the release of nitrite without the formation of the nitroso derivative(s). Results suggest that two types of enzymes were involved in HMX metabolism: one for denitration and the second for reduction to the nitroso derivative(s).     

Arabidopsis formin AtFH6 is a plasma membrane-associated protein upregulated in giant cells induced by parasitic nematodes

Plant-parasitic nematodes Meloidogyne spp induce an elaborate permanent feeding site characterized by the redifferentiation of root cells into multinucleate and hypertrophied giant cells. We have isolated by a promoter trap strategy an Arabidopsis thaliana formin gene, AtFH6, which is upregulated during giant cell formation. Formins are actin-nucleating proteins that stimulate de novo polymerization of actin filaments. We show here that three type-I formins were upregulated in giant cells and that the AtFH6 protein was anchored to the plasma membrane and uniformly distributed. Suppression of the budding defect of the Saccharomyces cerevisiae bni1Delta bnr1Delta mutant showed that AtFH6 regulates polarized growth by controlling the assembly of actin cables. Our results suggest that AtFH6 might be involved in the isotropic growth of hypertrophied feeding cells via the reorganization of the actin cytoskeleton. The actin cables would serve as tracks for vesicle trafficking needed for extensive plasma membrane and cell wall biogenesis. Therefore, determining how plant parasitic nematodes modify root cells into giant cells represents an attractive system to identify genes that regulate cell growth and morphogenesis.     

The structure of Acidithiobacillus ferrooxidans c(4)-cytochrome: a model for complex-induced electron transfer tuning

The study of electron transfer between the copper protein rusticyanin (RCy) and the c(4)-cytochrome CYC(41) of the acidophilic bacterium Acidithiobacillus ferrooxidans has evidenced a remarkable decrease of RCy's redox potential upon complex formation. The structure of the CYC(41) obtained at 2.2 A resolution highlighted a specific glutamate residue (E121) involved in zinc binding as potentially playing a central role in this effect, required for the electron transfer to occur. EPR and stopped-flow experiments confirmed the strong inhibitory effect of divalent cations on CYC(41):RCy complex formation. A docking analysis of the CYC(41) and RCy structure allows us to propose a detailed model for the complex-induced tuning of electron transfer in agreement with our experimental data, which could be representative of other copper proteins involved in electron transfer.