Conjugation is a major route of horizontal gene transfer, the driving force in the evolution of bacterial genomes. Antibiotic producing soil bacteria of the genus Streptomyces transfer DNA in a unique process involving a single plasmid-encoded protein TraB and a double-stranded DNA molecule. However, the molecular function of TraB in directing DNA transfer from a donor into a recipient cell is unknown. Here, we show that TraB constitutes a novel conjugation system that is clearly distinguished from DNA transfer by a type IV secretion system. We demonstrate that TraB specifically recognizes and binds to repeated 8 bp motifs on the conjugative plasmid. The specific DNA recognition is mediated by helix a3 of the C-terminal winged-helix-turn-helix domain of TraB. We show that TraB assembles to a hexameric ring structure with a central B3.1 nm channel and forms pores in lipid bilayers. Structure, sequence similarity and DNA binding characteristics of TraB indicate that TraB is derived from an FtsK-like ancestor protein, suggesting that Streptomyces adapted the FtsK/SpoIIIE chromosome segregation system to transfer DNA between two distinct Streptomyces cells.
Background: Histones organize the genomic DNA of eukaryotes into chromatin. The four core histone subunits consist of two consecutive helix-strand-helix motifs and are interleaved into heterodimers with a unique fold. We have searched for the evolutionary origin of this fold using sequence and structure comparisons, based on the hypothesis that folded proteins evolved by combination of an ancestral set of peptides, the antecedent domain segments.
Background: The AAA ATPases of the PAN/Rpt1–6 group regulate access of substrates to the 20S proteasome.Results: Two groups of AAA proteins, CDC48 and AMA, function as novel proteasomal ATPases in archaea.Conclusion: This network of regulatory ATPases increases the capacity of proteasomal protein degradation in archaea.Significance: Diversification at the level of the regulatory ATPase provides a contrast to the fully differentiated 26S proteasome of eukaryotes.
Most bacteria with a rod-shaped morphology contain an actin-like cytoskeleton consisting of MreB polymers, which form helical spirals underneath the cytoplasmic membrane to direct peptidoglycan synthesis for the elongation of the cell wall. In contrast, MreB of Streptomyces coelicolor is not required for vegetative growth but has a role in sporulation. Besides MreB, S. coelicolor encodes two further MreB-like proteins, Mbl and SCO6166, whose function is unknown. Whereas MreB and Mbl are highly similar, SCO6166 is shorter, lacking the subdomains IB and IIB of actin-like proteins. Here, we showed that MreB and Mbl are not functionally redundant but cooperate in spore wall synthesis. Expression analysis by semiquantitative reverse transcription-PCR revealed distinct expression patterns. mreB and mbl are induced predominantly during morphological differentiation. In contrast, sco6166 is strongly expressed during vegetative growth but switched off during sporulation. All genes could be deleted without affecting viability. Even a ⌬mreB ⌬mbl double mutant was viable. ⌬sco6166 had a wild-type phenotype. ⌬mreB, ⌬mbl, and ⌬mreB ⌬mbl produced swollen, prematurely germinating spores that were sensitive to various kinds of stress, suggesting a defect in spore wall integrity. During aerial mycelium formation, an Mbl-mCherry fusion protein colocalized with an MreB-enhanced green fluorescent protein (MreB-eGFP) fusion protein at the sporulation septa. Whereas MreB-eGFP localized properly in the ⌬mbl mutant, Mbl-mCherry localization depended on the presence of a functional MreB protein. Our results revealed that MreB and Mbl cooperate in the synthesis of the thickened spore wall, while SCO6166 has a nonessential function during vegetative growth.The peptidoglycan (PG) layer, consisting of long glycan strands cross-linked by short peptides, is a major determinant of bacterial cell shape (51). Most species with a complex, nonspherical morphology contain the actin-like MreB protein, which belongs to the HSP70-actin-sugar kinase (ASHKA) superfamily of proteins (4, 48). MreB was shown to polymerize into a dynamic helical filament underneath the cytoplasmic membrane spanning the long axis of the cells (11,17,25,30,33). In rod-shaped bacteria, MreB is thought to interact with other proteins to position a cell wall-synthesizing complex at the lateral cell wall (16,17,32). The incorporation of new PG at the lateral wall results in cell elongation, thus determining rod-shaped morphology. Gram-negative bacteria seem to have a single mreB gene, usually in an operon with mreC and mreD.
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