The control of asymmetric gene expression during Caulobacter cell differentiation

Species radiation by DNA replication that systematically exchanges nucleotides?

RNA and DNA syntheses share many properties. Therefore, the existence of 'swinger' RNAs, presumed 'orphan' transcripts matching genomic sequences only if transcription systematically exchanged nucleotides, suggests replication producing swinger DNA. Transcripts occur in many short-lived copies, the few cellular DNA molecules are long-lived. Hence pressures for functional swinger DNAs are greater than for swinger RNAs. Protein coding properties of swinger sequences differ from original sequences, suggesting rarity of corresponding swinger DNA. For genes producing structural RNAs, such as tRNAs and rRNAs, three exchanges (A<->T, C<->G and A<->T+C<->G) conserve self-hybridization properties. All nuclear eukaryote swinger DNA sequences detected in GenBank are for rRNA genes assuming A<->T+C<->G exchanges. In brachyuran crabs, 25 species had A<->T+C<->G swinger 18S rDNA, all matching the reverse-exchanged version of regular 18S rDNA of a related species. In this taxon, swinger replication of 18S rDNA apparently associated with, or even resulted in species radiation. A<->T+C<->G transformation doesn't invert sequence direction, differing from inverted repeats. Swinger repeats (detectable only assuming swinger transformations, A<->T+C<->G swinger repeats most frequent) within regular human rRNAs, independently confirm swinger polymerizations for most swinger types. Swinger replication might be an unsuspected molecular mechanism for ultrafast speciation.

Modes of cytometric bacterial DNA pattern: a tool for pursuing growth

Analyses of DNA pattern provide an excellent tool to determine activity states of bacteria. Bacterial cell cycle behaviour is generally different from the eukaryotic one and is pre-determined by the bacteria's diversity within the phylogenetic tree, and their metabolic traits. As a result, every species creates its specific proliferation pattern that differs from every other one. Up to now, just few bacterial species have been investigated and little information is available concerning DNA cycling even in already known species. This prevents understanding of the complexity and diversity of ongoing bacterial interactions in many ecosystems or in biotechnology. Flow cytometry is the only possible technique to shed light on the dynamics of bacterial communities and DNA patterns will help to unlock the hidden principles of their life. This review provides basic knowledge about the molecular background of bacterial cell cycling, discusses modes of cell cycle phases and presents techniques to both obtain DNA patterns and to combine the contained information with physiological cell states.

Transcriptional regulation of the S-layer protein type I secretion system inCaulobacter crescentus

The Gram-negative Caulobacter crescentus exports RsaA, the crystalline S-layer subunit protein using a dedicated type I secretion system. The protein and two transporter genes (rsaADE) are located together, comparable to the Escherichia coli type I hemolysin hlyCABD operon, where read through of a stem loop following hlyCA results in reduced transcription of the hlyBD. Using two genetic approaches and a direct assessment of transcription from regions 5' to the genes we learned that rsaD and rsaE were transcribed together as a separate transcript from rsaA. These results are contrary to previous assumptions about the rsaADE type I secretion gene control and add another theme to the area of type I secretion transcription regulation. It may be that to accommodate the high levels of RsaA secretion, the type I transporters must be transcribed independently from rsaA.

Identification and inactivation of three group 2 sigma factor genes in Anabaena sp. strain PCC 7120

Three new Anabaena sp. strain PCC 7120 genes encoding group 2 alternative sigma factors have been cloned and characterized. Insertional inactivation of sigD, sigE, and sigF genes did not affect growth on nitrate under standard laboratory conditions but did transiently impair the abilities of sigD and sigE mutant strains to establish diazotrophic growth. A sigD sigE double mutant, though proficient in growth on nitrate and still able to differentiate into distinct proheterocysts, was unable to grow diazotrophically due to extensive fragmentation of filaments upon nitrogen deprivation. This double mutant could be complemented by wild-type copies of sigD or sigE, indicating some degree of functional redundancy that can partially mask phenotypes of single gene mutants. However, the sigE gene was required for lysogenic development of the temperate cyanophage A-4L. Several other combinations of double mutations, especially sigE sigF, caused a transient defect in establishing diazotrophic growth, manifested as a strong and prolonged bleaching response to nitrogen deprivation. We found no evidence for developmental regulation of the sigma factor genes. luxAB reporter fusions with sigD, sigE, and sigF all showed slightly reduced expression after induction of heterocyst development by nitrogen stepdown. Phylogenetic analysis of cyanobacterial group 2 sigma factor sequences revealed that they fall into several subgroups. Three morphologically and physiologically distant strains, Anabaena sp. strain PCC 7120, Synechococcus sp. strain PCC 7002, and Synechocystis sp. strain PCC 6803 each contain representatives of four subgroups. Unlike unicellular strains, Anabaena sp. strain PCC 7120 has three additional group 2 sigma factors that cluster in subgroup 2.5b, which is perhaps specific for filamentous or heterocystous cyanobacteria.

Polyomavirus small t antigen prevents retinoic acid-induced retinoblastoma protein hypophosphorylation and redirects retinoic acid-induced G0 arrest and differentiation to apoptosis

Polyomavirus small t antigen (ST) impedes late features of retinoic acid (RA)-induced HL-60 myeloid differentiation as well as growth arrest, causing apoptosis instead. HL-60 cells were stably transfected with ST. ST slowed the cell cycle, retarding G2/M in particular. Treated with RA, the ST transfectants continued to proliferate and underwent apoptosis. ST also impeded the normally RA-induced hypophosphorylation of the retinoblastoma tumor suppressor protein consistent with failure of the cells to arrest growth. The RA-treated transfectants expressed CD11b, an early cell surface differentiation marker, but inducible oxidative metabolism, a later and more mature functional differentiation marker, was largely inhibited. Instead, the cells underwent apoptosis. ST affected significant known components of RA signaling that result in G0 growth arrest and differentiation in wild-type HL-60. ST increased the basal amount of activated ERK2, which normally increases when wild-type cells are treated with RA. ST caused increased RARalpha expression, which is normally down regulated in RA-treated wild-type cells. The effects of ST on RA-induced myeloid differentiation did not extend to monocytic differentiation and G0 arrest induced by 1,25-dihydroxy vitamin D3, whose receptor is also a member of the steroid-thyroid hormone superfamily. In this case, ST abolished the usually induced G0 arrest and retarded, but did not block, differentiation without inducing apoptosis, thus uncoupling growth arrest and differentiation. In sum, the data show that ST disrupted the normal RA-induced program of G0 arrest and differentiation, causing the cells to abort differentiation and undergo apoptosis.

Helicobacter pylori motility

Motility is essential for Helicobacter pylori colonization. This review discusses the biochemistry, genetics and genomics of the H. pylori flagellum, and compares these features with well-characterized bacteria.

Chromosome methylation and measurement of faithful, once and only once per cell cycle chromosome replication in Caulobacter crescentus

Caulobacter crescentus exhibits cell-type-specific control of chromosome replication and DNA methylation. Asymmetric cell division yields a replicating stalked cell and a nonreplicating swarmer cell. The motile swarmer cell must differentiate into a sessile stalked cell in order to replicate and execute asymmetric cell division. This program of cell division implies that chromosome replication initiates in the stalked cell only once per cell cycle. DNA methylation is restricted to the predivisional cell stage, and since DNA synthesis produces an unmethylated nascent strand, late DNA methylation also implies that DNA near the replication origin remains hemimethylated longer than DNA located further away. In this report, both assumptions are tested with an engineered Tn5-based transposon, Tn5Omega-MP. This allows a sensitive Southern blot assay that measures fully methylated, hemimethylated, and unmethylated DNA duplexes. Tn5Omega-MP was placed at 11 sites around the chromosome and it was clearly demonstrated that Tn5Omega-MP DNA near the replication origin remained hemimethylated longer than DNA located further away. One Tn5Omega-MP placed near the replication origin revealed small but detectable amounts of unmethylated duplex DNA in replicating stalked cells. Extra DNA synthesis produces a second unmethylated nascent strand. Therefore, measurement of unmethylated DNA is a critical test of the "once and only once per cell cycle" rule of chromosome replication in C. crescentus. Fewer than 1 in 1,000 stalked cells prematurely initiate a second round of chromosome replication. The implications for very precise negative control of chromosome replication are discussed with respect to the bacterial cell cycle.