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Martin Thanbichler

Publications and source records attributed to Martin Thanbichler.

10 recordsLinked to original sources
PubMed2006

MipZ, a spatial regulator coordinating chromosome segregation with cell division in Caulobacter.

Correct positioning of the division plane is a prerequisite for the generation of daughter cells with a normal chromosome complement. Here, we present a mechanism that coordinates assembly and placement of the FtsZ cytokinetic ring with bipolar localization of the newly duplicated chromosomal origins in Caulobacter. After replication of the polarly located origin region, one copy moves rapidly to the opposite end of the cell in an MreB-dependent manner. A previously uncharacterized essential protein, MipZ, forms a complex with the partitioning protein ParB near the origin of replication and localizes with the duplicated origin regions to the cell poles. MipZ directly interferes with FtsZ polymerization, thereby restricting FtsZ ring formation to midcell, the region of lowest MipZ concentration. The cellular localization of MipZ thus serves the dual function of positioning the FtsZ ring and delaying formation of the cell division apparatus until chromosome segregation has initiated.

Adenosine Triphosphatases↗
PubMed2006

Chromosome organization and segregation in bacteria.

In the recent years, considerable advances have been made towards understanding the structure and function of the bacterial chromosome. A number of different factors appear to cooperate in condensing DNA into a highly dynamic assembly of supercoiled loops. Despite this variability in the lower levels of chromatin structure, the global arrangement of chromosomal DNA within the cell is surprisingly conserved, with loci being arrayed along the cellular long axis in line with their order on the genomic map. This conserved pattern is propagated during the course of DNA segregation. First, after entry into S-phase, the newly synthesized origin regions are segregated in an active and directed process, involving the bacterial actin homolog MreB. Subsequent DNA segments then follow by different mechanisms. They are separated immediately after release from the replisome and move rapidly to their conserved positions in the incipient daughter cell compartments. Partitioning of the bacterial chromosome thus takes place while DNA replication is in progress.

Bacteria↗
PubMed2005

The bacterial nucleoid: a highly organized and dynamic structure.

Recent advances in bacterial cell biology have revealed unanticipated structural and functional complexity, reminiscent of eukaryotic cells. Particular progress has been made in understanding the structure, replication, and segregation of the bacterial chromosome. It emerged that multiple mechanisms cooperate to establish a dynamic assembly of supercoiled domains, which are stacked in consecutive order to adopt a defined higher-level organization. The position of genetic loci on the chromosome is thereby linearly correlated with their position in the cell. SMC complexes and histone-like proteins continuously remodel the nucleoid to reconcile chromatin compaction with DNA replication and gene regulation. Moreover, active transport processes ensure the efficient segregation of sister chromosomes and the faithful restoration of nucleoid organization while DNA replication and condensation are in progress.

Bacteria↗
PubMed2005

The structure and function of the bacterial chromosome.

Advances in microscopic and cell biological techniques have considerably improved our understanding of bacterial chromosome organization and dynamics. The nucleoid was formerly perceived to be an amorphous entity divided into ill-defined domains of supercoiling that are randomly deposited in the cell. Recent work, however, has demonstrated a remarkable degree of spatial organization. A highly ordered chromosome structure, established while DNA replication and partitioning are in progress, is maintained and propagated during growth. Duplication of the chromosome and partitioning of the newly generated daughter strands are interwoven processes driven by the dynamic interplay between the synthesis, segregation and condensation of DNA. These events are intimately coupled with the bacterial cell cycle and exhibit a previously unanticipated complexity reminiscent of eukaryotic systems.

Bacteria↗
PubMed2005

The choreographed dynamics of bacterial chromosomes.

Despite decades of study, the exquisite temporal and spatial organization of bacterial chromosomes has only recently been appreciated. The direct visualization of specific chromosomal loci has revealed that bacteria condense, move and position their chromosomes in a reproducible fashion. The realization that bacterial chromosomes are actively translocated through the cell suggests the existence of specific mechanisms that direct this process. Here, we review bacterial chromosome dynamics and our understanding of the mechanisms that direct and coordinate them.

Bacillus subtilis↗
PubMed2004

Selenocysteine tRNA-specific elongation factor SelB is a structural chimaera of elongation and initiation factors.

In all three kingdoms of life, SelB is a specialized translation elongation factor responsible for the cotranslational incorporation of selenocysteine into proteins by recoding of a UGA stop codon in the presence of a downstream mRNA hairpin loop. Here, we present the X-ray structures of SelB from the archaeon Methanococcus maripaludis in the apo-, GDP- and GppNHp-bound form and use mutational analysis to investigate the role of individual amino acids in its aminoacyl-binding pocket. All three SelB structures reveal an EF-Tu:GTP-like domain arrangement. Upon binding of the GTP analogue GppNHp, a conformational change of the Switch 2 region in the GTPase domain leads to the exposure of SelB residues involved in clamping the 5' phosphate of the tRNA. A conserved extended loop in domain III of SelB may be responsible for specific interactions with tRNA(Sec) and act as a ruler for measuring the extra long acceptor arm. Domain IV of SelB adopts a beta barrel fold and is flexibly tethered to domain III. The overall domain arrangement of SelB resembles a 'chalice' observed so far only for initiation factor IF2/eIF5B. In our model of SelB bound to the ribosome, domain IV points towards the 3' mRNA entrance cleft ready to interact with the downstream secondary structure element.

Amino Acid Sequence↗
PubMed2004

Rapid and sequential movement of individual chromosomal loci to specific subcellular locations during bacterial DNA replication.

The chromosomal origin and terminus of replication are precisely localized in bacterial cells. We examined the cellular position of 112 individual loci that are dispersed over the circular Caulobacter crescentus chromosome and found that in living cells each locus has a specific subcellular address and that these loci are arrayed in linear order along the long axis of the cell. Time-lapse microscopy of the location of the chromosomal origin and 10 selected loci in the origin-proximal half of the chromosome showed that during DNA replication, as the replisome sequentially copies each locus, the newly replicated DNA segments are moved in chronological order to their final subcellular destination in the nascent half of the predivisional cell. Thus, the remarkable organization of the chromosome is being established while DNA replication is still in progress. The fact that the movement of these 10 loci is, like that of the origin, directed and rapid, and occurs at a similar rate, suggests that the same molecular machinery serves to partition and place many, if not most, chromosomal loci at defined subcellular sites.

Caulobacter crescentus↗
PubMed2003

Purification and characterization of hexahistidine-tagged elongation factor SelB.

The cotranslational incorporation of selenocysteine into proteins is mediated by a specialized elongation factor, named SelB. Its amino-terminal three domains show homology to elongation factor EF-Tu and accordingly bind GTP and selenocysteyl-tRNASec. In addition, SelB exhibits a long carboxy-terminal extension that interacts with a secondary structure of selenoprotein mRNAs (SECIS element) positioned immediately downstream of the in-frame UGA codons specifying the sites of selenocysteine insertion. In this report, a fast and efficient method for the purification of large amounts of hexahistidine-tagged SelB is presented. After two chromatographic steps, 10 mg pure protein was isolated from 12 g wet cell pellet. Biochemical analysis of the purified protein showed that the tag does not influence the interaction of SelB with guanine nucleotides, SECIS elements, and selenocysteyl-tRNASec. In addition, the fusion protein is fully functional in mediating UGA read-through in vivo. It therefore represents an excellent model for studying the function of SelB and the mechanisms of selenocysteine incorporation.

Amino Acid Sequence↗
PubMed2002

The function of SECIS RNA in translational control of gene expression in Escherichia coli.

The incorporation of selenocysteine into proteins is directed by specific UGA codons and mRNA secondary structures, designated SECIS elements. In bacteria, these elements are positioned within the reading frame of selenoprotein mRNAs immediately downstream of the triplet coding for selenocysteine, and they tether a complex of the selenocysteine-specific elongation factor SelB, GTP and selenocysteyl-tRNA(Sec) to the site of UGA decoding. A SECIS-like structure was identified in the 5' non-translated region of the selAB transcript, encoding selenocysteine synthase and SelB. It specifically binds to SelB and the formation of a SelB.GTP.selenocysteyl-tRNA(Sec) complex on the SECIS-like element represses expression of the downstream gene. This effect is abolished by mutations preventing formation of the complex. The regulatory pattern observed correlated with the levels of sel gene products. As quaternary complex formation on the SECIS-like element did not influence the transcription rate and only slightly reduced the level of selAB mRNA, it was concluded that the structure is involved in regulating translation initiation efficiency, thereby coupling selenocysteine biosynthesis to the availability of the trace element selenium.

Bacterial Proteins↗