Phosphate and carbohydrate facilitate the formation of filamentous Salmonella enterica during osmotic stress.

Salmonella enterica is a human pathogen that can produce filamentous cells in response to environmental stress. The molecular mediators and biosynthetic pathways that contribute to the formation of filamentous cells (>10 µm in length) during osmotic stress are mostly unknown. The comparison of filamentous and non-filamentous cells in this study was aided by the use of a filtration step to separate cell types. Osmotic stress caused an efflux of phosphate from cells, and the addition of phosphate and a carbohydrate to Luria broth with 7 % NaCl (LB-7NaCl) significantly increased the proportion of filamentous cells in the population (58 %). In addition to direct measurements of intracellular and extracellular phosphate concentrations, the relative abundance of the iraP transcript that is induced by phosphate limitation was monitored. Non-filamentous cells had a greater relative abundance of iraP transcript than filamentous cells. IraP also affects the stability of RpoS, which regulates the general stress regulon, and was detected in non-filamentous cells but not filamentous cells. Markers of metabolic pathways for the production of acetyl-CoA (pflB, encoding for pyruvate formate lyase) and fatty acids (fabH) that are essential to membrane biosynthesis were found in greater abundance in filamentous cells than non-filamentous cells. There were no differences in the DNA, protein and biomass levels in filamentous and non-filamentous cells after 48 h of incubation, although the filamentous cells produced significantly (P<0.05) more acetate. This study found that phosphate and carbohydrate enhanced the formation of filamentous cells during osmotic stress, and there were differences in key regulatory elements and markers of metabolic pathways in filamentous and non-filamentous S. enterica.

Characterization of osmotically induced filaments of Salmonella enterica.

Salmonella enterica forms aseptate filaments with multiple nucleoids when cultured in hyperosmotic conditions. These osmotic-induced filaments are viable and form single colonies on agar plates even though they contain multiple genomes and have the potential to divide into multiple daughter cells. Introducing filaments that are formed during osmotic stress into culture conditions without additional humectants results in the formation of septa and their division into individual cells, which could present challenges to retrospective analyses of infectious dose and risk assessments. We sought to characterize the underlying mechanisms of osmotic-induced filament formation. The concentration of proteins and chromosomal DNA in filaments and control cells was similar when standardized by biomass. Furthermore, penicillin-binding proteins in the membrane of salmonellae were active in vitro. The activity of penicillin-binding protein 2 was greater in filaments than in control cells, suggesting that it may have a role in osmotic-induced filament formation. Filaments contained more ATP than did control cells in standardized cell suspensions, though the levels of two F(0)F(1)-ATP synthase subunits were reduced. Furthermore, filaments could septate and divide within 8 h in 0.2 × Luria-Bertani broth at 23°C, while nonfilamentous control cells did not replicate. Based upon the ability of filaments to septate and divide in this diluted broth, a method was developed to enumerate by plate count the number of individual, viable cells within a population of filaments. This method could aid in retrospective analyses of infectious dose of filamented salmonellae.

The latent membrane protein 1 (LMP1) oncogene of Epstein-Barr virus can simultaneously induce and inhibit apoptosis in B cells.

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) regulates its own expression and the expression of human genes via its two functional moieties; the transmembrane domains of LMP1 are required to regulate its expression via the unfolded protein response (UPR) and autophagy in B cells, and the carboxy-terminal domain of LMP1 activates cellular signaling pathways that affect cellular proliferation and survival. An apparent anomaly in the complex regulation of the UPR and autophagy by LMP1 is that the induction of either pathway can lead to cellular death, yet neither EBV-infected B cells nor B cells expressing only LMP1 die. Thus, we sought to understand how B cells that express LMP1 survive. The transmembrane domains of LMP1 activated apoptosis in B cells, the apoptosis required the UPR, and the carboxy-terminal domain of LMP1 blocked this apoptosis. The expression of the mRNA of Bcl2a1, encoding an antiapoptotic homolog of BCL2, correlated directly with the expression of LMP1 in EBV-positive B-cell strains, and its expression inhibited the apoptosis induced by the transmembrane domains of LMP1. These findings illustrate how the carboxy-terminal domain of LMP1 supports survival of B cells in the presence of the deleterious effects of the complex regulation of this viral oncogene.

How human tumor viruses make use of autophagy.

Viruses commandeer regulatory pathways of their hosts to optimize their success as cellular parasites. The human tumor viruses, Epstein-Barr Virus (EBV), Kaposi's Sarcoma Herpesvirus (KSHV), Hepatitis B Virus (HBV), and Hepatitis C Virus (HCV) all affect autophagy for their own ends. EBV and KSHV regulate it during latent infections, a phase when no progeny virus is produced, while HBV and HCV use autophagy to promote their productive infections. Here we shall compare and contrast how these human tumor viruses regulate autophagy and what they gain by the appropriation of this cellular pathway.

The microRNAs of Epstein-Barr Virus are expressed at dramatically differing levels among cell lines.

Epstein-Barr Virus (EBV) encodes multiple microRNAs (miRNAs) from two primary transcripts, BHRF1 and the BARTs. The expression of BHRF1 miRNAs is dependent on the type of viral latency, whereas the BART miRNAs are expressed in cells during all forms of latency. It is not known how these miRNAs are otherwise regulated, though. We have used quantitative, stem-loop, real-time PCR to measure the expression of EBV's miRNAs and found them to differ nearly 50- and 25-fold among all tested cell lines and among EBV-positive Burkitt's lymphomas, respectively. In addition, the expression of individual BART miRNAs within a cell can differ by 50-fold or more despite the fact these miRNAs are likely transcribed together as a single primary transcript. These measurements are illuminating: they indicate that few of EBV's miRNAs are expressed at levels comparable to those of cellular miRNAs in most cell lines and therefore likely function interdependently.

Mutual interdependence of MSI1 (CAC3) and YAK1 in Saccharomyces cerevisiae.

The MSI1 (CAC3) gene of Saccharomyces cerevisiae has been implicated in diverse cellular functions, including suppression of the RAS/cAMP/protein kinase A signaling pathway, chromatin assembly and transcriptional co-repression. Seeking to identify the molecular mechanisms by which Msi1p carries out these distinct activities, a novel genetic interaction was uncovered with YAK1, which encodes a kinase that antagonizes the RAS/cAMP pathway. MSI1 was capable of efficiently suppressing the heat shock sensitivity caused by deletion of yak1. Surprisingly, the YAK1 gene is required for Msi1p to associate with Cac1p in the yeast two-hybrid system. A new activity of Msi1p was identified: the ability to activate transcription of a reporter gene when tethered near the promoter, but only in the absence of fermentable carbon sources. This transcriptional activation function was diminished substantially by the loss of YAK1. Furthermore, MSI1 influences YAK1 function; over-expression of YAK1 decreased the growth rate, but only in the presence of a functional MSI1 gene. Finally, it is shown that YAK1 antagonizes nuclear accumulation of Msi1p in non-fermenting cells. Taken together, these data demonstrate a novel interaction between Msi1p and Yak1p in which each protein influences the activity of the other.