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Smc-ScpAB complexes, also called bacterial condensin, are comprised of highly conserved proteins present in a wide variety of bacteria and archaea. They facilitate bacterial chromosome segregation probably by organizing and/or compacting nascent sister chromosomes. In some γ-proteobacteria however the Smc-ScpAB complex appears to have been replaced by a different SMC complex, called MukBEF. Petrushenko et al., (2011) now show in this issue of Molecular Microbiology that distantly related homologues of MukBEF, called MksBEF, can be found in the genomes of many more species of bacteria (such as Pseudomonas aeruginosa and Bacillus cereus) that seem to be scattered over the phylogenetic tree. The newly identified MksBEF proteins are organized in operons like MukBEF and occur in combination with Smc-ScpAB and/or MukBEF. Mutations in mksBEF in P. aeruginosa result in the formation of anucleate cells whereas its mild overexpression can suppress the accumulation of anucleate cells in smc null mutants. These findings suggest that MksBEF and Smc-ScpAB share their duties in chromosome partitioning in P. aeruginosa and possibly in other bacteria too. Thus, chromosome organization and segregation in bacteria appear to be governed by up to three distinct condensin complexes, indicating that this process could be much more sophisticated than previously appreciated., (© 2011 Blackwell Publishing Ltd.)

Proper segregation of DNA replication products is essential in all cells. In Bacillus subtilis, two protein complexes have been implicated in this process: the ParAB homologs, Soj and Spo0J, and the bacterial Smc/ScpAB complex, also called condensin. Here we demonstrate that Smc is highly enriched in the region around the origin of replication, specifically near parS sites to which Spo0J binds and at highly transcribed genes. Furthermore, we find that efficient recruitment of Smc to a large region around the origin of replication depends on the presence of Spo0J. We show that Spo0J performs two independent functions: regulation of initiation of DNA replication via Soj and promotion of chromosome segregation by Smc recruitment. Our results demonstrate a direct functional interaction between two widely conserved systems involved in chromosome replication and segregation.

Cohesin is a multisubunit complex that mediates sister-chromatid cohesion. Its Smc1 and Smc3 subunits possess ABC-like ATPases at one end of 50 nm long coiled coils. At the other ends are pseudosymmetrical hinge domains that interact to create V-shaped Smc1/Smc3 heterodimers. N- and C-terminal domains within cohesin's kleisin subunit Scc1 bind to Smc3 and Smc1 ATPase heads respectively, thereby creating a huge tripartite ring. It has been suggested that cohesin associates with chromosomes by trapping DNA within its ring. Opening of the ring due to cleavage of Scc1 by separase destroys sister-chromatid cohesion and triggers anaphase. We show that cohesin's hinges are not merely dimerization domains. They are essential for cohesin's association with chromosomes, which is blocked by artificially holding hinge domains together but not by preventing Scc1's dissociation from SMC ATPase heads. Our results suggest that entry of DNA into cohesin's ring requires transient dissociation of Smc1 and Smc3 hinge domains.

In this study, we developed a method to improve the delineation of intrinsic brain tumors based on the changes in metabolism due to tumor infiltration. Proton magnetic resonance spectroscopic imaging ((1)H-MRSI) with a nominal voxel size of 0.45 cm(3) was used to investigate the spatial distribution of choline-containing compounds (Cho), creatine (Cr) and N-acetyl-aspartate (NAA) in brain tumors and normal brain. Ten patients with untreated gliomas were examined on a 1.5 T clinical scanner using a MRSI sequence with PRESS volume preselection. Metabolic maps of Cho, Cr, NAA and Cho/NAA ratios were calculated. Tumors were automatically segmented in the Cho/NAA images based on the assumption of Gaussian distribution of Cho/NAA values in normal brain using a limit for normal brain tissue of the mean + three times the standard deviation. Based on this threshold, an area was calculated which was delineated as pathologic tissue. This area was then compared to areas of hyperintense signal caused by the tumor in T2-weighted MRI, which were determined by a region growing algorithm in combination with visual inspection by two experienced clinicians. The area that was abnormal on (1)H-MRSI exceeded the area delineated via T2 signal changes in the tumor (mean difference 24%) in all cases. For verification of higher sensitivity of our spectroscopic imaging strategy we developed a method for coregistration of MRI and MRSI data sets. Integration of the biochemical information into a frameless stereotactic system allowed biopsy sampling from the brain areas that showed normal T2-weighted signal but abnormal (1)H-MRSI changes. The histological findings showed tumor infiltration ranging from about 4-17% in areas differentiated from normal tissue by (1)H-MRSI only. We conclude that high spatial resolution (1)H-MRSI (nominal voxel size = 0.45 cm(3)) in combination with our segmentation algorithm can improve delineation of tumor borders compared to routine MRI tumor diagnosis.

Chromosome segregation during mitosis and meiosis depends on the linkage of sister DNA molecules after replication. These links, known as sister-chromatid cohesion, are provided by a multi-subunit complex called cohesin. Recent papers suggest that chromatin-remodeling complexes also have a role in the generation of sister-chromatid cohesion. It remains unclear whether they do so by facilitating the recruitment of cohesin to specific chromosomal sequences or by modifying an event at replication forks giving rise to cohesion between sister DNAs.

The cohesin complex is essential for sister chromatid cohesion during mitosis. Its Smc1 and Smc3 subunits are rod-shaped molecules with globular ABC-like ATPases at one end and dimerization domains at the other connected by long coiled coils. Smc1 and Smc3 associate to form V-shaped heterodimers. Their ATPase heads are thought to be bridged by a third subunit, Scc1, creating a huge triangular ring that could trap sister DNA molecules. We address here whether cohesin forms such rings in vivo. Proteolytic cleavage of Scc1 by separase at the onset of anaphase triggers its dissociation from chromosomes. We show that N- and C-terminal Scc1 cleavage fragments remain connected due to their association with different heads of a single Smc1/Smc3 heterodimer. Cleavage of the Smc3 coiled coil is sufficient to trigger cohesin release from chromosomes and loss of sister cohesion, consistent with a topological association with chromatin.

In persistent hyperinsulinemic hypoglycemia of infancy, ketone body concentrations are abnormally low at times of hypoglycemia, depriving the brain of its most important alternative fuel. The neuroprotective effect of endogenous ketone bodies is evidenced by animal and human studies, but knowledge about exogenous supply is limited. Assuming that exogenous ketone body compounds as a dietetic food might replace this alternative energy source for the brain, we have monitored the fate of orally supplemented DL sodium beta-hydroxybutyrate (beta-OHB) in two 6-mo-old infants with persistent hyperinsulinemic hypoglycemia for 5 and 7 mo, while on frequent tube-feedings and treatment with octreotide. Near total (95%) pancreatectomy had been ineffective in one patient and was refused in the other. In blood, concentrations of beta-OHB increased to levels comparable to a 16- to 24-h fast while on DL sodium beta-OHB 880 to 1000 mg/kg per day. In cerebrospinal fluid, concentrations of beta-OHB increased to levels comparable to a 24- to 40-h fast, after single dosages of 4 and 8 g, respectively. High ratios of beta-OHB to acetoacetate indicated exogenous origin of beta-OHB. An increase of intracerebral concentrations of beta-OHB could be demonstrated by repetitive single-voxel proton magnetic resonance spectroscopy by a clear doublet at 1.25 ppm. Oral DL sodium beta-OHB was tolerated without side effects. This first report on oral supplementation of DL sodium beta-OHB in two patients with persistent hyperinsulinemic hypoglycemia demonstrates effective uptake across the blood-brain barrier and could provide the basis for further evaluation of the neuroprotective effect of beta-OHB in conditions with hypoketotic hypoglycemia.