Several parameters of phage T4 adsorption to and growth in Escherichia coli B/r were determined. All changed monotonously with the bacterial growth rate (p), which was modified by nutritional conditions. Adsorption rate was faster at higher p values, positively correlated to cell size, and increased by pretreatment with low penicillin (Pn) concentrations; it was directly proportional to total cellular surface area, indicating a constant density of 14 receptors on cell envelopes irrespective of growth conditions. Parameters of phage development and cell lysis were pdependent. The rate of phage release and burst size increased, while the eclipse and latent periods decreased with increasing p. Differentiation between the contribution of several physiological parameters to the development of 14 was performed by manipulating the host cells. A competitive inhibitor of glucose uptake, methyl a-o-glucoside, was exploited to reduce the growth rate in the same effective carbon source. Synchronous cells were obtained by the 'baby-machine' and large cells were obtained by pretreatment with low Pn concentrations. Lysis was delayed by superinfection, and DNA content and concentration were modified by growing a thy mutant in limiting thymine concentrations. The results indicate that burst size is not limited by cell size or DNA composition, nor directly by the rate of metabolism, but rather by the rates of synthesis and assembly of phage components and by lysis time. The rates of synthesis and assembly of phage components seem to depend on the content of the protein-synthesizing system and lysis time seems to depend on cellular dimensions.

Summary The histone deacetylase SIRT6 promotes DNA repair, but its activity declines with age, with a concomitant accumulation of DNA damage. Furthermore, SIRT6 knockout mice exhibit an accelerated aging phenotype and die prematurely. Here, we report that brain-specific SIRT6-deficient mice survive, but present behavioral defects with major learning impairments by 4 months of age. Moreover, the brains of these mice show increased signs of DNA damage, cell death and hyperphosphorylated Tau – a critical mark in several neurodegenerative diseases. Mechanistically, SIRT6 regulates Tau protein stability and phosphorylation through increased activation of the kinase GSK3α/β. Finally, SIRT6 mRNA and protein levels are reduced in patients with Alzheimer’s disease. Taken together, our results suggest that SIRT6 is critical to maintain genomic stability in the brain and its loss leads to toxic Tau stability and phosphorylation. Therefore, SIRT6 and its downstream signaling could be targeted in Alzheimer’s disease and age related neurodegeneration.

2A variety of very basic questions asked 30 to 40 years ago remain unanswered today. How do bacteria select and maintain a defined size, length, and width? How do they control the timing of chromosome replication? What is the mechanism by which they segregate their replicated (or replicating) chromosomes? How do they know where and when to divide? From the literature it is apparent that there is resurgence in studies aimed at answering these important questions. While much progress has been made, we still have a ways to go. This minireview revisits an old technique (thymine limitation) in a new light and illustrates how one can use this technique to manipulate some of the parameters of the cell cycle under balanced growth conditions, which should be helpful in addressing some of the above questions. Hopefully this review will spawn new interest and ideas about the old questions that can be tested by this technique.Ever since the first thymine-requiring (so-called thymineless) strain of Escherichia coli was isolated in 1947 (described in reference 94), thyA mutants have been employed to follow DNA synthesis in vivo (60). Since thymine is a precursor of DNA only, radioactive isotope-labeled thymine and scintillation counters are used to track synthesis in bacteria and their viruses. Before the semiconservative nature of replication predicted by the double-helix model was demonstrated, running density gradients of DNA labeled with the heavy thymine analogue 5Ј-bromodeoxyuridine had been considered (43). However, saving on radiolabeled thymine isotope by using exceedingly low concentrations during the labeling period (for example, see reference 61) led to some flawed conclusions and discrepancies (for example, see reference 55). These were resolved by systematic investigations of pool sizes of thymine metabolites and of the rate of chromosome replication in relation to the external concentrations of thymine (for example, see reference 85). Those studies and their usefulness in getting to understand the composition, structure, and function of the bacterial cell are summarized here. This issue is of particular current importance, because the distinction between the two completely different physiological states of "thymine starvation" and "thymine limitation" has become somewhat vague (for example, see references 24 and 134). It is crucial to realize that the former is a pathological state of the cell, whereas the latter is not (Table 1). Thymine limitation is used as a means to dissociate the rate of DNA replication from the culture growth rate, i.e., to change the relative schedule of the replication and division cycles. This technique to dissociate the two rates differs from the classical method of nutritional shifts, because it does not affect the major metabolic pathways prevailing in the cell. To understand this method, one must know the unique modes by which thymine is metabolized. The earlier review by Ahmad et al. (2) presents an excellent account of the metabolic pathways involving thymine for both prokaryote...

DNA double-strand breaks (DSB) are the most deleterious type of DNA damage. In this work, we show that SIRT6 directly recognizes DNA damage through a tunnel-like structure that has high affinity for DSB. SIRT6 relocates to sites of damage independently of signaling and known sensors. It activates downstream signaling for DSB repair by triggering ATM recruitment, H2AX phosphorylation and the recruitment of proteins of the homologous recombination and non-homologous end joining pathways. Our findings indicate that SIRT6 plays a previously uncharacterized role as a DNA damage sensor, a critical factor in initiating the DNA damage response (DDR). Moreover, other Sirtuins share some DSB-binding capacity and DDR activation. SIRT6 activates the DDR before the repair pathway is chosen, and prevents genomic instability. Our findings place SIRT6 as a sensor of DSB, and pave the road to dissecting the contributions of distinct DSB sensors in downstream signaling.

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