Productive T7 infection of Escherichia coli F+ cells and anucleate minicells

Intracellular kinetics of a growing virus: a genetically structured simulation for bacteriophage T7

Viruses have evolved to efficiently direct the resources of their hosts toward their own reproduction. A quantitative understanding of viral growth will help researchers develop antiviral strategies, design metabolic pathways, construct vectors for gene therapy, and engineer molecular systems that self-assemble. As a model system we examine here the growth of bacteriophage T7 in Escherichia coli using a chemical-kinetic framework. Data published over the last three decades on the genetics, physiology, and biophysics of phage T7 are incorporated into a genetically structured simulation that accounts for entry of the T7 genome into its host, expression of T7 genes, replication of T7 DNA, assembly of T7 procapsids, and packaging of T7 DNA to finally produce intact T7 progeny. Good agreement is found between the simulated behavior and experimental observations for the shift in transcription capacity from the host to the phage, the initiation times of phage protein synthesis, and the intracellular assembly of both wild-type phage and a fast-growing deletion mutant. The simulation is utilized to predict the effect of antisense molecules targeted to different T7 mRNA. Further, a postulated mechanism for the down regulation of T7 transcription in vivo is quantitatively examined and shown to agree with available data. The simulation is found to be a useful tool for exploring and understanding the dynamics of virus growth at the molecular level. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 375-389, 1997.

Immunization with non-replicating E. coli minicells delivering both protein antigen and DNA protects mice from lethal challenge with lymphocytic choriomeningitis virus

In the midst of new investigations into the mechanisms of both delivery and protection of new vaccines and vaccine carriers, it has become clear that immunization with delivery mechanisms that do not involve living, replicating organisms are vastly preferred. In this report, non-replicating bacterial minicells simultaneously co-delivering the nucleoprotein (NP) of lymphocytic choriomeningitis virus (LCMV) and the corresponding DNA vaccine were tested for the ability to generate protective cellular immune responses in mice. It was found that good protection (89%) was achieved after intramuscular administration, moderate protection (31%) was achieved after intranasal administration, and less protection (7%) was achieved following gastric immunization. These results provide a solid foundation on which to pursue the use of bacterial minicells as a non-replicating vaccine delivery platform.

Immune responses elicited by bacterial minicells capable of simultaneous DNA and protein antigen delivery

Recent events surrounding emerging infectious diseases, bioterrorism and increasing multidrug antibiotic resistance in bacteria have drastically increased current needs for effective vaccines. Many years of study have shown that live, attenuated pathogens are often more effective at delivering heterologous protein or DNA to induce protective immune responses. However, these vaccine carriers have inherent safety concerns that have limited their development and their use in many patient populations. Studies using nonliving delivery mechanisms have shown that providing both protein antigen and DNA encoding the antigen to an individual induces an improved, more protective immune response but rarely, if ever, are both delivered simultaneously. Here, non-replicating bacterial minicells derived from a commensal E. coli strain are shown to effectively induce antigen-specific immune responses after simultaneous protein and DNA delivery. These data demonstrate the potential use of achromosomal bacterial minicells as a vaccine carrier.

Effects of Escherichia coli physiology on growth of phage T7 in vivo and in silico

Phage development depends not only upon phage functions but also on the physiological state of the host, characterized by levels and activities of host cellular functions. We established Escherichia coli at different physiological states by continuous culture under different dilution rates and then measured its production of phage T7 during a single cycle of infection. We found that the intracellular eclipse time decreased and the rise rate increased as the growth rate of the host increased. To develop mechanistic insight, we extended a computer simulation for the growth of phage T7 to account for the physiology of its host. Literature data were used to establish mathematical correlations between host resources and the host growth rate; host resources included the amount of genomic DNA, pool sizes and elongation rates of RNA polymerases and ribosomes, pool sizes of amino acids and nucleoside triphosphates, and the cell volume. The in silico (simulated) dependence of the phage intracellular rise rate on the host growth rate gave quantitatively good agreement with our in vivo results, increasing fivefold for a 2.4-fold increase in host doublings per hour, and the simulated dependence of eclipse time on growth rate agreed qualitatively, deviating by a fixed delay. When the simulation was used to numerically uncouple host resources from the host growth rate, phage growth was found to be most sensitive to the host translation machinery, specifically, the level and elongation rate of the ribosomes. Finally, the simulation was used to follow how bottlenecks to phage growth shift in response to variations in host or phage functions.

Cytoplasmic RNA Polymerase in Escherichia coli

To obtain an estimate for the concentration of free functional RNA polymerase in the bacterial cytoplasm, the content of RNA polymerase beta and beta' subunits in DNA-free minicells from the minicell-producing Escherichia coli strain chi925 was determined. In bacteria grown in Luria-Bertani medium at 2.5 doublings/h, 1.0% of the total protein was RNA polymerase. The concentration of cytoplasmic RNA polymerase beta and beta' subunits in minicells produced by this strain corresponded to about 17% (or 2.5 microM) of the value found in whole cells. Literature data suggest that a similar portion of cytoplasmic RNA polymerase subunits is in RNA polymerase assembly intermediates and imply that free functional RNA polymerase can form a small percentage of the total functional enzyme in the cell. On infection with bacteriophage T7, 20% of the minicells produced progeny phage, whereas infection in 80% of the cells was abortive. RNA polymerase subunits in lysozyme-freeze-thaw lysates of minicells were associated with minicell envelopes and were without detectable activity in an in vitro transcription assay. Together, these results suggest that most functional RNA polymerase is associated with the DNA and that little if any segregates into DNA-free minicells.

Aberrant regulation of synthesis and degradation of viral proteins in coliphage lambda-infected UV-irradiated cells and in minicells

The patterns of bacteriophage lambda proteins synthesized in UV-irradiated Escherichia coli cells and in anucleate minicells are significantly different; both systems exhibit aberrations of regulation in lambda gene expression. In unirradiated cells or cells irradiated with low UV doses (less than 600 J/m2), regulation of lambda protein synthesis is controlled by the regulatory proteins CI, N, CII, CIII, Cro, and Q. As the UV dose increases, activation of transcription of the cI, rexA, and int genes by CII and CIII proteins fails to occur and early protein synthesis, normally inhibited by the action of Cro, continues. After high UV doses (greater than 2,000 J/m2), late lambda protein synthesis does not occur. Progression through the sequence of regulatory steps in lambda gene expression is slower in infected minicells. In minicells, there is no detectable cII- and cIII-dependent synthesis of CI, RexA, or Int proteins and inhibition of early protein synthesis by Cro activity is always incomplete. The synthesis of early b region proteins is not subject to control by CI, N, or Cro proteins, and evidence is presented suggesting that, in minicells, transcription of the early b region is initiated at a promoter(s) within the b region. Proteolytic cleavage of the regulatory proteins O and N and of the capsid proteins C, B, and Nu3 is much reduced in infected minicells. Exposure of minicells to very high UV doses before infection does not completely inhibit late lambda protein synthesis.

Bacteriophage phi 29 infection of Bacillus subtilis minicells

Bacteriophage phi 29 can infect B. subtilis minicells and synthesize all the phage-coded proteins detected in ultraviolet irradiated-infected B. subtilis cells. Synthesis of phage unit-length DNA has been obtained after infection of minicells with phi 29. The DNA can be encapsulated in particles with a sedimentation rate similar to that of phage phi 29 produced in B. subtilis cells. The particles produced in minicells can be adsorbed to B. subtilis cells, but infectivity has not been demonstrated because of the very low burst-size obtained.

Radiation sensitivity of messenger RNA

Messenger RNA function is inactivated by irradiation with ultraviolet light. A unit length mRNA (in bases) is 2-3 times more sensitive than a unit length of DNA (in base pairs) with respect to the inactivation of template function. These data stem from four experimental systems all of which do not repair DNA: the translation of E. coli mRNA in rifampicin-treated cells, of T7 mRNA in infected E. coli, of f2 phage RNA in vivo, and of stable mRNA in chromosomeless minicells. The comparison of relative sensitivities to UV is relevant to the technique of UV mapping of transcription units which enjoys increasing popularity in pro- and eukaryotic genetic research.