Nuclear lamina dysfunction triggers a germline stem cell checkpoint

LEM domain (LEM-D) proteins are conserved components of the nuclear lamina (NL) that contribute to stem cell maintenance through poorly understood mechanisms. The Drosophila emerin homolog Otefin (Ote) is required for maintenance of germline stem cells (GSCs) and gametogenesis. Here, we show that ote mutants carry germ cell-specific changes in nuclear architecture that are linked to GSC loss. Strikingly, we found that both GSC death and gametogenesis are rescued by inactivation of the DNA damage response (DDR) kinases, ATR and Chk2. Whereas the germline checkpoint draws from components of the DDR pathway, genetic and cytological features of the GSC checkpoint differ from the canonical pathway. Instead, structural deformation of the NL correlates with checkpoint activation. Despite remarkably normal oogenesis, rescued oocytes do not support embryogenesis. Taken together, these data suggest that NL dysfunction caused by Otefin loss triggers a GSC-specific checkpoint that contributes to maintenance of gamete quality.

Otefin is a nuclear lamina protein required for survival of Drosophila germ stem cells. Here the authors show that nuclear lamina dysfunction resulting from loss of Otefin activates a DNA damage-independent germ stem cell-specific checkpoint, mediated by the ATR and Chk2 kinases, which ensures that healthy gametes are passed on to the next generation.

Regulatory inputs for the synthesis of ComK, the competence transcription factor of Bacillus subtilis

Competence in Bacillus subtilis is expressed post-exponentially in response to signals which are interpreted by a complex network of regulatory proteins. This network culminates in the transcriptional activation of a set of late-competence proteins that mediate DNA binding and uptake during transformation. ComK, a protein that binds to competence promoters and appears to activate their transcription, is itself synthesized in response to the signal-transduction network. ComK is known to be required for the transcription of its own gene. We have placed comK under control of the xylose-inducible PxylA promoter and used this construct to show that ComK synthesis is sufficient as well as necessary to induce competence. We have also confirmed that the Mec proteins act post-transcriptionally to inactivate ComK, probably by protein-protein interaction. We have further demonstrated that ComS is required to generate an upstream signal that causes reversal of Mec-induced inactivation of ComK. In addition to ComK itself, DegU, AbrB, and SinR are required for comK transcription; mutations in their genes are bypassed by PxylA-comK induction, and therefore their products appear not to act via the Mec proteins. Overproduction of ComK, in a loss-of-function mec mutant, is also known to bypass the need for DegU, SinR and AbrB. We propose that these proteins enhance the activity of ComK as a positive autoregulatory transcription factor, acting as coactivator proteins when ComK is present at low concentrations. Finally, we demonstrate that when ComK is synthesized from the PxylA promoter and mecA is inactivated by mutation, no additional growth-stage-regulated control of competence can be detected.

Polynucleotide phosphorylase is necessary for competence development in Bacillus subtilis

comR (pnpA) is a newly identified gene in Bacillus subtilis that is necessary for the expression of late competence genes. Transformability of a comR (pnpA) mutant is 1-5% of that seen in comR+ strains. Cloning and sequencing identified ComR as polynucleotide phosphorylase (PNPase). The PNPase amino acid sequence has 50% identity and 67% similarity with the Escherichia coli enzyme. Enzymatic assays show that this is the only PNPase activity in B. subtilis. comR (pnpA) is necessary for comG-lacZ and comK-lacZ expression, but this requirement is bypassed by a mecA disruption. In B. subtilis, the loss of PNPase has little effect on expression from a fusion of the srfA promoter directly to lacZ, but is necessary for normal expression from certain srfA-lacZ fusions that include portions of the normal srfA transcript. When a srfA-lacZ translational fusion is tested in isogenic pnpA+ and pnpA derivatives of E. coli, lower expression is seen in the pnpA mutant. Since expression from lacZ fusions to comA, sinR, and mecA appeared similar in the B. subtilis pnpA and pnpA+ strains, the loss of PNPase does not have a strong general effect on gene expression. These results suggest that PNPase may be necessary for modification of the srfA transcript in order to activate translation or stabilize the transcript, and that this may be necessary for competence development. This is the first evidence of post-transcriptional effects on the development of competence in B. subtilis.

comK encodes the competence transcription factor, the key regulatory protein for competence development in Bacillus subtilis

comK is a positive autoregulatory gene occupying a central position in the competence-signal-transduction network. All regulatory routes identified in this network converge at the level of comK expression. The ComK protein is required for the transcriptional induction of comK and the late competence genes, which specify morphogenetic and structural proteins necessary for construction of the DNA-binding and uptake apparatus. In this report we demonstrate that ComK specifically binds to DNA fragments containing promoter and upstream sequences of the genes it affects (comC, comE, comF, comG and comK). Using portions of the region upstream of comC we show that the ComK-binding sequences are essential for the expression of competence. Moreover, we demonstrate that the presence of ComK stimulates the expression of comF-lacZ and comG-lacZ translational fusions in vivo in Escherichia coli. These results indicate that the gene product of comK is identical to the previously inferred competence transcription factor (CTF).

The twisted 'life' of DNA in the cell: bacterial topoisomerases

DNA topoisomerases are essential to the cell for the regulation of DNA supercoiling levels and for chromosome decatenation. The proposed mechanisms for these reactions are essentially the same, except that a change in supercoiling is due to an intramolecular event, while decatenation requires an intermolecular event. The characterized bacterial topoisomerases appear capable of both types of reaction in vitro. Four DNA topoisomerases have been identified in Escherichia coli. Topoisomerase I, gyrase, and topoisomerase IV normally appear to have distinct essential functions within the cell. Gyrase and topoisomerase I are responsible for the regulation of DNA supercoiling. Both gyrase and topoisomerase IV are necessary for chromosomal decatenation. Multiple topoisomerases with distinct functions may give the cell more precise control over DNA topology by allowing tighter regulation of the principal enzymatic activities of these different proteins.