Degradation of Escherichia coli B deoxyribonucleic acid after infection with deoxyribonucleic acid-defective amber mutants of bacteriophage T7

States of phage T3/T7 capsids: buoyant density centrifugation and cryo-EM

Mature double-stranded DNA bacteriophages have capsids with symmetrical shells that typically resist disruption, as they must to survive in the wild. However, flexibility and associated dynamism assist function. We describe biochemistry-oriented procedures used to find previously obscure flexibility for capsids of the related phages, T3 and T7. The primary procedures are hydration-based buoyant density ultracentrifugation and purified particle-based cryo-electron microscopy (cryo-EM). We review the buoyant density centrifugation in detail. The mature, stable T3/T7 capsid is a shell flexibility-derived conversion product of an initially assembled procapsid (capsid I). During DNA packaging, capsid I expands and loses a scaffolding protein to form capsid II. The following are observations made with capsid II. (1) The in vivo DNA packaging of wild type T3 generates capsid II that has a slight (1.4%), cryo-EM-detected hyper-expansion relative to the mature phage capsid. (2) DNA packaging in some altered conditions generates more extensive hyper-expansion of capsid II, initially detected by hydration-based preparative buoyant density centrifugation in Nycodenz density gradients. (3) Capsid contraction sometimes occurs, e.g., during quantized leakage of DNA from mature T3 capsids without a tail.

Flap endonuclease of bacteriophage T7: Possible roles in RNA primer removal, recombination and host DNA breakdown

Gene 6 protein of bacteriophage T7 has 5′-3′-exonuclease activity specific for duplex DNA. We have found that gene 6 protein also has flap endonuclease activity. The flap endonuclease activity is considerably weaker than the exonuclease activity. Unlike the human homolog of gene 6 protein, the flap endonuclease activity of gene 6 protein is dependent on the length of the 5′-flap. This dependency of activity on the length of the 5′-flap may result from the structured helical gateway region of gene 6 protein which differs from that of human flap endonuclease 1. The flap endonuclease activity provides a mechanism by which RNA-terminated Okazaki fragments, displaced by the lagging strand DNA polymerase, are processed. 3′-extensions generated during degradation of duplex DNA by the exonuclease activity of gene 6 protein are inhibitory to further degradation of the 5′-terminus by the exonuclease activity of gene 6 protein. The single-stranded DNA binding protein of T7 overcomes this inhibition.

Flap endonuclease activity of gene 6 exonuclease of bacteriophage T7

Flap endonucleases remove flap structures generated during DNA replication. Gene 6 protein of bacteriophage T7 is a 5'-3'-exonuclease specific for dsDNA. Here we show that gene 6 protein also possesses a structure-specific endonuclease activity similar to known flap endonucleases. The flap endonuclease activity is less active relative to its exonuclease activity. The major cleavage by the endonuclease activity occurs at a position one nucleotide into the duplex region adjacent to a dsDNA-ssDNA junction. The efficiency of cleavage of the flap decreases with increasing length of the 5'-overhang. A 3'-single-stranded tail arising from the same end of the duplex as the 5'-tail inhibits gene 6 protein flap endonuclease activity. The released flap is not degraded further, but the exonuclease activity then proceeds to hydrolyze the 5'-terminal strand of the duplex. T7 gene 2.5 single-stranded DNA-binding protein stimulates the exonuclease and also the endonuclease activity. This stimulation is attributed to a specific interaction between the two proteins because Escherichia coli single-stranded DNA binding protein does not produce this stimulatory effect. The ability of gene 6 protein to remove 5'-terminal overhangs as well as to remove nucleotides from the 5'-termini enables it to effectively process the 5'-termini of Okazaki fragments before they are ligated.

Genomically recoded organisms expand biological functions

We describe the construction and characterization of a genomically recoded organism (GRO). We replaced all known UAG stop codons in Escherichia coli MG1655 with synonymous UAA codons, which permitted the deletion of release factor 1 and reassignment of UAG translation function. This GRO exhibited improved properties for incorporation of nonstandard amino acids that expand the chemical diversity of proteins in vivo. The GRO also exhibited increased resistance to T7 bacteriophage, demonstrating that new genetic codes could enable increased viral resistance.

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.

Genome function--a virus-world view

By studying viruses one may begin to understand how static genomes can define dynamic processes of development. This talk will describe some of the approaches we are taking, using computer simulations and laboratory experiments, to account for the many molecular-level processes and interactions that occur when a common bacterium, E. coli, is infected by one of its viruses, phage T7. We accounted for processes of phage genome entry, transcription, translation, and DNA replication, including protein-DNA and protein-protein regulatory interactions, and we predicted the dynamics of phage progeny formation. The simulations have enabled us to identify limiting host-cell resources in phage growth, discover novel anti-viral strategies, and suggest frameworks for mining data from global mRNA and protein studies.

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.

Role of exonuclease in the specificity of bacteriophage T7 DNA packaging

During morphogenesis in vivo, bacteriophage T7 packages and cuts to mature size an end-to-end concatemer of its nonpermuted, terminally repetitious, double-stranded, mature DNA. Efficient production (90-100%) and packaging (20-35%) of concatemers has also been demonstrated in extracts of T7-infected cells (in vitro) (Son, M., Hayes, S. J., and Serwer, P. [1988] Virology 162, 38-46). By use of both this procedure of in vitro DNA packaging and in-gel hybridization to packaged DNA fractionated by agarose gel electrophoresis, the specificity of packaging in vitro is found to depend on the presence of T7 gene 6 exonuclease (p6). In the absence of p6 in vitro, no concatemerization is detected and packaging of DNA nonhomologous to T7 DNA (bacteriophage P22 DNA) is as efficient (0.05-1.1%) as the packaging of monomeric T7 DNA. Addition of p6 in vitro both stimulates the concatemerization-packaging of T7 DNA and suppresses the packaging of P22 DNA. The packaging efficiency for concatemeric T7 DNA is 29-611 x higher than that for monomeric T7 DNA. Inhibition of the packaging of P22 DNA by p6 is correlated with the formation of single-stranded P22 DNA ends. These data are explained by the hypothesis that a DNA molecule with a single-stranded end is packaged less efficiently than the same DNA without the single-stranded end. Testing this hypothesis in vivo reveals that both p6 and gene 3 endonuclease contribute to suppressing the packaging of host DNA.

Bacteriophage lysis: mechanism and regulation

Bacteriophage lysis involves at least two fundamentally different strategies. Most phages elaborate at least two proteins, one of which is a murein hydrolase, or lysin, and the other is a membrane protein, which is given the designation holin in this review. The function of the holin is to create a lesion in the cytoplasmic membrane through which the murein hydrolase passes to gain access to the murein layer. This is necessary because phage-encoded lysins never have secretory signal sequences and are thus incapable of unassisted escape from the cytoplasm. The holins, whose prototype is the lambda S protein, share a common organization in terms of the arrangement of charged and hydrophobic residues, and they may all contain at least two transmembrane helical domains. The available evidence suggests that holins oligomerize to form nonspecific holes and that this hole-forming step is the regulated step in phage lysis. The correct scheduling of the lysis event is as much an essential feature of holin function as is the hole formation itself. In the second strategy of lysis, used by the small single-stranded DNA phage phi X174 and the single-stranded RNA phage MS2, no murein hydrolase activity is synthesized. Instead, there is a single species of small membrane protein, unlike the holins in primary structure, which somehow causes disruption of the envelope. These lysis proteins function by activation of cellular autolysins. A host locus is required for the lytic function of the phi X174 lysis gene E.

Initiation of DNA replication at cloned origins of bacteriophage T7

Bacteriophage T7 DNA replication is initiated at a site 15% of the distance from the genetic left end of the chromosome. This primary origin contains two tandem T7 RNA polymerase promoters (phi 1.1A and phi 1.1B) followed by an A + T-rich region. When the primary origin region is deleted replication initiates at secondary origins. We have analyzed the ability of plasmids containing cloned fragments of T7 to replicate after infection of Escherichia coli with bacteriophage T7. All cloned T7 fragments that support plasmid replication contain a T7 promoter but a T7 promoter alone is not sufficient for replication. Replication of plasmids containing the primary origin is dependent on T7 DNA polymerase and gene 4 protein (helicase/primase) and a portion of the A + T-rich region. The other T7 fragments that support plasmid replication after T7 infection are promoter regions phi OR, phi 13 and phi 6.5 (secondary origins). When both the primary and secondary origins are present simultaneously on compatible plasmids, replication of each is temporally regulated. Such regulation may play a role during T7 DNA replication.

Multidimensional analysis of intracellular bacteriophage T7 DNA: effects of amber mutations in genes 3 and 19

By use of rate-zonal centrifugation, followed by either one- or two-dimensional agarose gel electrophoresis, the forms of intracellular bacteriophage T7 DNA produced by replication, recombination, and packaging have been analyzed. Previous studies had shown that at least some intracellular DNA with sedimentation coefficients between 32S (the S value of mature T7 DNA) and 100S is concatemeric, i.e., linear and longer than mature T7 DNA. The analysis presented here confirmed that most of this DNA is linear, but also revealed a significant amount of circular DNA. The data suggest that these circles are produced during DNA packaging. It is proposed that circles are produced after a capsid has bound two sequential genomes in a concatemer. The size distribution of the linear, concatemeric DNA had peaks at the positions of dimeric and trimeric concatemers. Restriction endonuclease analysis revealed that most of the mature T7 DNA subunits of concatemers were joined left end to right end. However, these data also suggest that a comparatively small amount of left-end to left-end joining occurs, possibly by blunt-end ligation. A replicating form of T7 DNA that had an S value greater than 100 (100S+ DNA) was also found to contain concatemers. However, some of the 100S+ DNA, probably the most branched component, remained associated with the origin after agarose gel electrophoresis. It has been found that T7 protein 19, known to be required for DNA packaging, was also required to prevent loss, probably by nucleolytic degradation, of the right end of all forms of intracellular T7 DNA. T7 gene 3 endonuclease, whose activity is required for both recombination of T7 DNA and degradation of host DNA, was required for the formation of the 32S to 100S molecules that behaved as concatemers during gel electrophoresis. In the absence of gene 3 endonuclease, the primary accumulation product was origin-associated 100S+ DNA with properties that suggest the accumulation of branches, primarily at the left end of mature DNA subunits within the 100S+ DNA.

Relative roles of T7 RNA polymerase and gene 4 primase for the initiation of T7 phage DNA replication in vivo

Initiation sites of T7 phage DNA replication in the presence and absence of T7 phage gene 4 primase have been analyzed by using Escherichia coli cells infected with T7 phage amber mutants, T73,6 and T73,4,6, respectively. Restriction analysis of the [3H]thymidine-labeled DNA, synthesized by the T73,4,6 phage-infected cells in the presence of 2',3'-dideoxy-3'-azidothymidine, has shown that only the light (L) strand of T7 DNA has been synthesized from the primary origin area to the right. Transition sites from RNA to DNA have been located precisely in the primary origin region of the T7 phage genome. In the gene 4- condition, greater than 20 transition sites have been detected only in the L strand. They scattered widely downstream from the phi 1.1 promoters and mostly downstream from the phi 1.3 promoter. The same transition sites have been detected in the gene 4+ condition, suggesting that the transcripts started from these promoters are used as primers of the rightward L-strand DNA synthesis in the gene 4+ condition. In addition, many heavy (H)- and L-strand transition sites have been detected at gene 4 primase sites in the gene 4+ condition. The relative roles of T7 phage RNA polymerase and primase at the primary origin have been discussed.

Gene 3 endonuclease of bacteriophage T7 resolves conformationally branched structures in double-stranded DNA

Gene 3 endonuclease of bacteriophage T7 has been expressed from the cloned gene, purified, and characterized as to its activity on different DNA substrates. Besides its known strong preference for cutting single-stranded DNA rather than double-stranded DNA, the enzyme has a strong preference for cutting conformationally branched structures in double-stranded DNA, either X or Y-shaped branches. Three types of branched DNA substrates were used: relaxed circular DNAs containing large cruciform structures (a model for Holliday structures, presumed intermediates in genetic recombination); X-shaped molecules having a limited potential for branch migration, made from the cloned phage and bacterial arms of the lambda attachment site; and Y-shaped molecules, made by hybridizing molecules homologous except for a 2 X 21 base-pair palindrome in one of them. Gene 3 endonuclease cuts two opposing strands at or near the branchpoint to resolve these substrates into linear molecules, and does not cut the potentially single-stranded tips of the stem-and-loop structure generated from the palindrome. The position of the cleavage points on the equivalent arm of two X-shaped molecules, constructed from wild-type and mutant lambda attachment sites, show that the enzyme can cut at several different sites within or slightly 5' of the limited region of branch migration. The various activities of gene 3 endonuclease are consistent with the known role of this enzyme in genetic recombination, in maturation and packaging of T7 DNA, and in degradation of host DNA, and suggest that the enzyme recognizes a specific structural feature in DNA. Its cleavage specificity, ready availability, and ability to act at physiological pH and ionic conditions may make gene 3 endonuclease useful as a probe for specific DNA structures or for binding of proteins that alter DNA structure.

Mechanism of inhibition of bacteriophage T7 DNA synthesis in Escherichia coli B cells infected by alkylated bacteriophage T7

Quantitative analysis of DNA replication, in E. coli B cells infected by methyl methanesulfonate-treated bacteriophage T7, showed that production of phage DNA was delayed and decreased. The cause of the delay appeared to be a delay in host-DNA breakdown, the process which provides nucleotides for phage-DNA synthesis. In addition, reutilisation of host-derived nucleotides was impaired. These observations can be accounted for by a model in which methyl groups on phage DNA slow down DNA injection and also reduce the replicational template activity of the DNA once it has entered the cell. Repair of alkylated phage DNA may be required not only for replication but also for normal injection of DNA.

Bacteriophage T7 defective in the gene 6 exonuclease promotes site-specific cleavages of T7 DNA in vivo and in vitro

Site-specific cleavages of intracellular DNA were demonstrated in bacteriophage T7 6am-infected cells. The sites of the cleavages were located at 46.8 and 68.7% (1% of the T7 DNA length = 400 base pairs) from the left end of the T7 genome. These cleavages required the products of genes 3 (endonuclease), 4 (DNA primase), and 5 (DNA polymerase). However, the product of gene 6 (exonuclease) must be absent. Site-specific cleavage was also shown to occur in vitro in extracts of T7 6am-infected cells, although at a different site: 82.8% from the left end of the T7 genome.

Genetic recombination of bacteriophage T7 in vivo studied by use of a simple physical assay

A new physical method was developed to assay genetic recombination of phage T7 in vivo. The assay utilized T7 mutants that carry unique restriction sites and was based on the detection of a new restriction fragment generated by recombination. Using this assay, we reexamined the genetic requirements for recombination of T7 DNA. Our results were in total agreement with previous findings in that recombination required the products of genes 3 (endonuclease), 4 (primase), 5 (DNA polymerase), and 6 (exonuclease). Recombination was found to be independent of DNA ligase and DNA packaging and maturation functions.

A novel mutant of bacteriophage T7 that is defective in early phage DNA synthesis

A mutant of bacteriophage T7 is described which produces smaller plaques than the wild type and is defective in early phage DNA synthesis. The mutation is located in the Class II transcriptional region of the T7 genome together with all the other genes involved in phage DNA synthesis, but it could not be placed into any of the existing known T7 genes. DNA replication in strain R9 begins at the same time as for wild type although it proceeds very slowly until 15 minutes after infection, after which time DNA synthesis is apparently normal. It is concluded therefore that there are two types of DNA replication in phage T7, which differ with respect to their dependence on the mutant function. The change from one mode to the other is marked by the formation of folded, complex DNA inside the cell.

Regulated breakdown of Escherichia coli deoxyribonucleic acid during intraperiplasmic growth of Bdellovibrio bacteriovorus 109J

During growth of Bdellovibrio bacteriovorus on [2-14C]deoxythymidine-labeled Escherichia coli, approximately 30% of the radioactivity was released to the culture fluid as nucleoside monophosphates and free bases; the remainder was incorporated by the bdellovibrio. By 60 min after bdellovibrio attack, when only 10% of the E. coli deoxyribonucleic acid (DNA) had been solubilized, the substrate cell DNA was degraded to 5 X 10(5)-dalton fragments retained within the bdelloplast. Kinetic studies showed these fragments were formed as the result of sequential accumulation of single- and then double-strand cuts. DNA fragments between 2 X 10(3) and 5 X 10(5) daltons were never observed. Chloramphenicol, added at various times after initiation of bdellovibrio intraperiplasmic growth on normal or on heated E. coli, which have inactivated deoxyribonucleases, inhibited further breakdown and solubilization of substrate cell DNA. Analysis of these intraperiplasmic culture deoxyribonuclease activities showed that bdellovibrio deoxyribonucleases are synthesized while E. coli nucleases are inactivated. It is concluded that continuous and sequential synthesis of bdellovibrio deoxyribonucleases of apparently differing specificities is necessary for complete breakdown and solubilization of substrate cell DNA, and that substrate cell deoxyribonucleases are not involved in any significant way in the degradation process.

Shutoff of host macromolecular synthesis after T-even bacteriophage infection

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Purification and structures of recombining and replicating bacteriophage T7 DNA

During the infection of Escherichia coli by bacteriophage T7, there is a gradual conversion of host DNA to T7 DNA. Recombination and replication occur during this time. We have devised a new way of examining the physical structures of the intermediates of these processes. It is based on the observation that there are no sites in T7 DNA susceptible to cleavage by the restriction endonuclease EcoRI. E. coli DNA, on the other hand, is susceptible to degradation by EcoRI. Thus, phage and host DNA can be separated by sucrose gradient centrifugation after treatment with EcoRI. Concatemeric T7 DNA contains a high proportion of branched, gapped, and whiskered structures. These appear to be intermediates of replication and recombination. This approach also monitors the conversion process from host to T7 DNA.

Properties of deoxynucleoside 5'-monophosphatase induced by bacteriophage T5 after infection of Escherichia coli

Bacteriophage T5 induced a deoxynucleoside 5'-monophosphatase during its infection of Escherichia coli. The enzyme was purified about 100-fold. It was clearly distinct from the host 5'-nucleotidase activity in its physical characteristics and substrate specificity. The enzyme was active on deoxynucleoside 5'-monophosphates but was not active as a phosphatase on ribonucleotides, deoxynucleoside 5'-triphosphates, deoxynucleoside 3'-monophosphates, or deoxyoligonucleotides. Furthermore, it did not have oligonucleotidase or exonuclease activity. The enzyme could exist in multimeric form but had a monomer molecular weight of about 25,000.

RNA-linked nascent DNA pieces in T7 phage-infected Escherichia coli cells. I. Role of gene 6 exonuclease in removal of the linked RNA

The presence of RNA-linked nascent DNA pieces in T7 phage-infected Escherichia coli cells has been shown by the selective degradation of the 5'-hydroxyl-terminated nascent DNA, produced by alkali or RNase treatment, with spleen exonuclease. At 43 degrees C, the proportion of RNA-linked DNA pieces in nascent short dna is 50 to 60% in T7 ts136 (ts mutant of gene 6) phage-infected E. coli, whereas that in T7 wild-type phage-infected cells is less than 6%. Joining of the nascent pieces is greatly retarded in T7 ts136-infected E. coli temperature sensitive polA mutants at 43 degrees C. These results suggest that gene 6 exonuclease plays a role in removal of the linked RNA during the discontinuous replication of T7 DNA.

Intracellular organization of bacteriophage T7 DNA: analysis of parenteral bacteriophage T7 DNA-membrane and DNA-protein complexes

After infection of Escherichia coli with bacteriophage T7, the parenteral DNA forms a stable association with host cell membranes. The DNA-membrane complex isolated in cesium chloride gradients is free of host DNA and the bulk of T7 RNA. The complex purified through two cesium chloride gradients contains a reproducible set of proteins which are enriched in polypeptides having molecular weights of 54,000, 34,000, and 32,000. All proteins present in the complex are derived from host membranes. Treatment of the complex with Bruij-58 removes 95% of the membrane lipid and selectively releases certain protein components. The Brij-treated complex has an S value of about 1,000 and the sedimentation rate of this material is not altered by treatment with Pronase or RNase.

Role of gene 2 in bacteriophage T7 DNA synthesis

Studies have been carried out to elucidate the in vivo function of gene 2 in T7 DNA synthesis. In gene 2-infected cells the rate of incorporation of (3-H)thymidine into acid-insoluble material is about 60% that of cells infected with T7 wild type. Gene 2 mutants do not however produce viable phage after infection of the nonpermissive host. In T7 wild type-infected cells, a major portion of the newly alkaline sucrose gradients. The concatemers serve as precursors for the formation of mature T7 DNA as demonstrated in pulse-chase experiments. In similar studies carried out with gene 2-infected cells, concatemers are not detected when the intracellular DNA is analyzed at several different times during the infection process. The DNA made during a gene 2 infection is present as duplex structures with a sedimentation rate close to mature T7 DNA.

Early events after infection of Escherichia coli by bacteriophage T5. Induction of a 5'-nucleotidase activity and excretion of free bases

Thymine-containing compounds, produced degradation of Escherichia coli DNA after infection of the cells with bacteriophage T5, did not accumulate in the cell but were excreted into the medium as the DNA was degraded. The ultimate degradation product was extracellular thymine that was not reutilized when T5 DNA synthesis began. This excretion of thymine may have been due in part to the induction of 5'-nucleotidase activity within 3 min after T5 infection. The level of this activity reached a maximum between 4 to 6 min after infection and then rapidly declined to its preinfection level by 10 to 15 min after infection. Chloramphenicol added before or soon after infection prevented the appearance of the nucleotidase. The induced nucleotidase activity was active not only on dTMP but also on dAMP, dGMP, and dCMP.

Isolation and characterization of prototrophic mutants of Escherichia coli unable to support the intracellular growth of T7

Mutants of E. coli B/1 were isolated which grew normally but did not permit the intracellular growth of bacteriophage T7. Two classes of mutants were studied in detail (tsnB(-) and tsnC(-)). These strains adsorbed T7 normally and were killed by the infection. Synthesis of T7 RNA and of early and late classes of T7 proteins occurred normally after infection. In T7-infected tsnB(-) cells, T7 DNA synthesis stopped prematurely shortly after its onset, suggesting that the tsnB function affects a step in the late phase of T7 DNA replication. Mutants of T7 were isolated (T7beta) which could grow on tsnB(-) cells. In T7-infected tsnC(-) cells, T7 DNA synthesis was completely blocked, suggesting that the tsnC function affects a step in an early phase of T7 DNA replication.

Point mutants in the D2a region of bacteriophage T4 fail to induce T4 endonuclease IV

We have studied the properties of presumptive point mutants in the D2a region of bacteriophage T4. Dominance tests showed that the D2a mutation was recessive to the wild-type allele. The mutations were shown to map in the D2a region by complementation against rII deletions. The D2a mutations were also located between gene 52 and rIIB by two- and three-factor crosses. The mutants are located at at least two distinct loci in the D2a region. The point mutants grow normally on all hosts tested and none of the mutants makes T4 endonuclease IV. We propose the name "denB" for the D2a locus.

Coiled rings of DNA released from cells infected with bacteriophages T7 or T4 or from uninfected Escherichia coli

The replicating intracellular DNA of phage T7 was labeled at high specific activity with tritiated thymidine. The DNA of uninfected Escherichia coli was similarly labeled. Portions of cells which contained replicating phage T7 or E. coli DNA were lysed by a lysozyme, freeze-thaw, sodium lauryl sulfate procedure, and the DNA was spread on Millipore membranes for visualization by autoradiography. The DNA of phage T7 appeared to be highly concatenated reaching lengths of up to 721 mum. Much of the DNA of phage T7 and E. coli was retained in compact globular structures. In addition, orderly coiled rings of varying diameter up to about 43 mum were regularly observed. Similar coiled ring structures were also observed in autoradiographs of replicating phage T4 DNA which had been prepared in previous experiments. Worcel and Burgi (27) have presented evidence that E. coli chromosomes, when gently extracted from cells, are in a multilooped and superhelically twisted configuration. The coiled rings which we have observed may correspond to the relaxed, multilooped configurations which they find when the superhelical twists have been relieved by one or more nicks in each loop.