Processing of bacteriophage lambda DNA during its assembly into heads

Structural basis of DNA packaging by a ring-type ATPase from an archetypal viral system

Many essential cellular processes rely on substrate rotation or translocation by a multi-subunit, ring-type NTPase. A large number of double-stranded DNA viruses, including tailed bacteriophages and herpes viruses, use a homomeric ring ATPase to processively translocate viral genomic DNA into procapsids during assembly. Our current understanding of viral DNA packaging comes from three archetypal bacteriophage systems: cos, pac and phi29. Detailed mechanistic understanding exists for pac and phi29, but not for cos. Here, we reconstituted in vitro a cos packaging system based on bacteriophage HK97 and provided a detailed biochemical and structural description. We used a photobleaching-based, single-molecule assay to determine the stoichiometry of the DNA-translocating ATPase large terminase. Crystal structures of the large terminase and DNA-recruiting small terminase, a first for a biochemically defined cos system, reveal mechanistic similarities between cos and pac systems. At the same time, mutational and biochemical analyses indicate a new regulatory mechanism for ATPase multimerization and coordination in the HK97 system. This work therefore establishes a framework for studying the evolutionary relationships between ATP-dependent DNA translocation machineries in double-stranded DNA viruses.

A microbial genetic journey

Fortunately, I began research in 1950 when the basic concepts of microbial genetics could be explored experimentally. I began with bacteriophage lambda and tried to establish the colinearity of its linkage map with its DNA molecule. My students and I worked out the regulation of lambda repressor synthesis for the establishment and maintenance of lysogeny. We also investigated the proteins responsible for assembly of the phage head. Using cell extracts, we discovered how to package DNA inside the head in vitro. Around 1972, I began to use molecular genetics to understand the developmental biology of Myxococcus xanthus. In particular, I wanted to learn how myxococcus builds its multicellular fruiting body within which it differentiates spores. We identified two cell-to-cell signals used to coordinate development. We have elucidated, in part, the signal transduction pathway for C-signal that directs the morphogenesis of a fruiting body.

Torsional dynamics and orientation of DNA--DAPI complexes

The flexibility of calf thymus DNA and several polynucleotides was measured using the anisotropy decay of DAPI bound to DNA, a minor groove probe. DNA torsional dynamics were analyzed using the Schurr model [Allison, S. A., & Schurr, J. M. (1979) Chem. Phys. 41, 35-44] in the infinite polymer length approximation. Time-resolved fluorescence depolarization was measured using a frequency-double mode-locked dye laser and frequency-domain acquisition methods. At very high P/D ratios, the anisotropy decay is dominated by DNA torsional dynamics. The recovered values of the torsional elastic constant were in good agreement with literature values obtained using other DNA probes. The exact knowledge of the angle between the probe emission dipole transition moment and the helix axis is critical for the determination of the polymer elastic constant. At low P/D ratios, energy transfer between dye molecules strongly contributes to the anisotropy decay. We have developed a statistical model that describes the anisotropy decay, when the correct geometrical factors are included. At low P/D ratios the anisotropy decay is dominated by fluorescence homotransfer. In this regime, it is possible to determine the orientation of the dye molecule with respect to the polymer with accuracy. The values obtained for the distance and orientation of the DAPI molecules in solution using the fluorescence measurements are in excellent agreement with those from the crystal structure of the oligonucleotides-DAPI complex by Dickerson's group [Larsen T.A., Goodsell, D. S., Cascio, D., Grzeskowiak, K., & Dickerson, R. E. (1989) J. Biomol. Struct. Dyn. 7, 477-491].

Ar+ plasma-induced damage to DNA in bacteriophage lambda: implications for the arrangement of DNA in the phage head

Bacteriophage lambda was bombarded with low-energy Ar+ ions with the goal of determining whether particular regions of the DNA genome are found preferentially in the outer portion of the packaged DNA mass. The strategy was to fragment the DNA selectively near the surface of the virus by exposing intact phage to Ar+ ions energetic enough to break covalent chemical bonds in DNA but not energetic enough to penetrate deeply beneath the viral capsid shell. Broken DNA was then isolated, and its genomic origin was identified by Southern hybridization to mapped restriction fragments of lambda DNA. Analysis of such Southern blots revealed that all regions of the lambda genome were represented among the small DNA fragments generated during all times of Ar+ bombardment examined. Depending on the duration of exposure, however, particular regions of the genome were found to be enriched in the small-fragment population. After short periods of exposure, sequences from the leftmost 10% and from the right half of the standard genetic map were enriched in the broken-DNA fraction. Among sequences in the right half of the genome, the enrichment was progressively more pronounced beginning in the middle of the genetic map and proceeding toward the right end. In phage bombarded for longer periods of time, rightward sequences were preferentially depleted in the small-fragment population. In contrast, when Ar+ bombardment was carried out with free lambda DNA rather than intact phage, small DNA fragments arose uniformly from all regions of the genome at all times of exposure examined. The results indicate that in the intact phage, DNA sequences from the right half and from the very leftmost regions of the genome have a tendency to lie closer to the capsid than does the remainder of the genome. Since DNA is packaged into the prohead beginning at the left end, our results suggest that packaging occurs in such a way that newly entering DNA tends to be disposed externally to that packaged at earlier times.

Construction and characterization of a NotI linking library of human chromosome 21

Effective procedures have been developed for constructing NotI linking libraries starting from chromosome-specific genomic libraries. Fifteen different single copy and two rDNA NotI linking clones from human chromosome 21 were identified in two libraries. Their chromosomal origin was confirmed, and regional location established using hybrid cell panels. Hybridization experiments with these probes revealed pairs of genomic NotI fragments, each ranging in size from less than 0.05 to 4.0 Mb. Many fragments displayed cell type variation. The total size of the NotI fragments detected in a human fibroblast cell line (GM6167) and mouse hybrid cell containing chromosome 21 as its only human component (WAV17) were approximately 32 and 34 Mb, respectively. If these fragments were all non-overlapping, this would correspond to about 70% of the 50-Mb content estimated for the whole chromosome. The linking clones will be enormously useful in the subsequent construction of a NotI restriction map of this chromosome. Characterization of these clones indicates the presence of numerous additional sites for other enzymes that recognize sequences containing CpG. Thus most NotI linking clones appear to derive from CpG islands and probably identify the 5' end of genes.

Ion etching of bacteriophage lambda: evidence that the right end of the DNA is located at the outside of the phage DNA mass

Bacteriophage lambda was etched in an Ar+ plasma under conditions in which the capsid and some of the DNA were eroded (by sputtering) from the particle surface. Analysis of the DNA remaining in etched phage demonstrated an enrichment in sequences derived from the left end and middle of the genome; sequences from the right end were selectively lost. The results suggest that the DNA in the mature phage is arranged with its left end toward the center and its right end toward the exterior of the overall DNA mass. Since the left end is the first to enter the phage prohead, the results are most compatible with the view that prohead filling also proceeds from the center to the exterior of the cavity. The suggested arrangement of lambda DNA is comparable to that observed in phage T4 and is consistent with the spiral-fold model of packaged DNA.

A coupled in vitro system for the formation and packaging of concatemeric phage T1 DNA

Extracts derived from E. coli cells infected non-permissively with phage T1 amber mutants were used in an in vitro system to investigate the packaging of T1 DNA into phage heads. The standard extract used infections with amber mutants in genes 1 and 2 (g1- g2-) which are defective in T1 DNA synthesis but can synthesis the proteins required for particle morphogenesis. g1- g2- extracts packaged T1+ virion DNA molecules with an efficiency of 3 X 10(5) pfu/micrograms DNA. Extracts from cells infected with phage also defective in DNA synthesis but carrying additional mutations in genes 3.5 or 4 which are required for concatemer formation in vivo (g1- g3.5- and g1- g4- extracts) package T1 virion DNA at substantially lower efficiencies. Analysis of the DNA products from these in vitro reaction showed that concatemeric DNA is formed very efficiently by g1- g2- extracts but not by g1- g3.5- or g1- g4- extracts. These results are interpreted as evidence that the T1 in vitro DNA packaging system primarily operates in a similar manner to the in vivo headful mechanism. This is achieved in vitro by the highly efficient conversion of T1 virion DNA into concatemers which are then packaged with a much lower efficiency into heads to form infectious particles. A secondary pathway for packaging T1 DNA into heads and unrelated to the headful mechanism may also exist.

Improved in vitro packaging of coliphage lambda DNA: a one-strain system free from endogenous phage

In previous systems for in vitro packaging of lambda DNA, phages are produced from the packaging components as well as from added DNA. We have developed a new genetic strategy for in vitro packaging that bypasses this endogenous phage problem. Our system employs a single bacterial strain whose lambda prophage codes for all of the packaging proteins but is deleted for cos, the packaging origin. Crude extracts of the single lysogen: (i) are virtually free from endogenous phages, (ii) package added lambda DNA efficiently and (iii) are easy to prepare. Using the cos- in vitro packaging system we show that packaging of lambda linear monomers is a second-order reaction, but that packaging from concatemers prepared by annealing or ligation is first order. We conclude that in our cos- system, linear monomers are a poor substrate for in vitro packaging but that packaging from concatemers works well.

Evidence for inclusion of regions of nonhomology in heteroduplex products of bacteriophage lambda recombination

Total intracellular DNA was isolated from replication-restricted bacteriophage lambda crosses in which the infecting parents were heteroallelic for wild-type and deletion mutant alleles. This DNA was examined for the presence of heteroduplex DNA molecules that contained wild-type sequences in one strand and deletion-mutant sequences in the other. Molecules hybrid for a 689-nucleotide deletion in the immunity region of lambda were detected at significant levels only in crosses in which both the red recombination system of lambda and the rec recombination system of Escherichia coli were active. Molecules hybrid for a 1300-nucleotide deletion in the central portion of the lambda genome were detected at significant levels in DNA isolated from both red+ and red- crosses in which recA function was present.

Biologically active recombinant formed through DNA pairing by purified recA protein in vitro

We have detected in vitro homologous recombination mediated by purified recA protein of Escherichia coli as a recombinant phage produced by using the DNA packaging system of phage lambda. When double-stranded DNA of phage lambda carrying amber mutations is incubated with double-stranded DNA carrying the wild-type genes in the presence of recA protein, Mg++ and ATP, and the DNA packaged, amber+ recombinant phage is produced at a high frequency. This reaction depends completely upon the function of the wild-type recA protein. After incubation of 32P-labeled linear DNA (Form III) with bromouracil-labeled circular DNA (Form I-Form II mixture) in the presence of recA protein, Mg++ and ATP, about 10% of the 32P-counts band at an intermediate density in CsCl equilibrium gradient. This fraction yields a high percentage of the recombinant phage after DNA packaging and shows the alpha-shaped and sigma-shaped joint molecules of linear and circular DNA under the electron microscope. Furthermore, we demonstrate that a non-homologous region inhibits the recombination reaction when it is between the marker concerned and the closer cos end. Our results indicate that recA protein acts directly in the initial step of recombination to join the homologous double-stranded DNA and that the resulting molecule can be matured into the recombinant DNA.

Lambda bacteriophage-mediated transduction of ColE1 deoxyribonucleic acid having a lambda bacteriophage-cohesive end site: selection of packageable-length deoxyribonucleic acid

An in vitro recombinant ColE1-cos lambda deoxyribonucleic acid (DNA) molecule, pKY96, has 70% of the length of lambda phage DNA. The process of lambda phage-mediated transduction of pKY96 generated a small amount of transducing phage particles containing ColE1-cos lambda DNA molecules of 80 or 101% of the length of lambda phage DNA, in addition to those containing original pKY96 DNA molecules. The newly isolated larger plasmid DNAs were transduced 100 times more efficiently than pKY96 DNA. Their structures were compared with that of a prototype pKY96 DNA, and the mechanism of the formation of these molecules is discussed.

178-Nucleotide sequence surrounding the cos site of bacteriophage lambda DNA

A nucleotide sequence of 61 nucleotides at the left end and 117 nucleotides at the right end of DNA from bacteriophage lambdacI857Sam7 was determined by the Maxam and Gilbert method. A perfect inverted repeat sequence of 10 nucleotides is near the left end, and one of 15 nucleotides is near the right end. DNA from another closely related lambda strain, lambdacI857prm116Sam7, has about 10% divergence in the sequence of the first 110 nucleotides at the right end and has a 17-member perfect inverted repeat sequence.

Packaging of ColE1 DNA having a lambda phage cohesive end site

The mechanism of lambda phage-mediated transduction of hybrid colicin E1 DNAs of various lengths was studied, and factors influencing the formation of these transducing particles were investigated. The results were as follows: 1. The presence of a cohesive end site of lambda phage (coslambda) on colicin E1 DNA was essential for packaging of the DNA. 2. Packaging of colicin E1 DNAs, which carry coslambda with molecular sizes corresponding to 68% of that of lambda phage DNA, was observed in the absence of all known recombination functions of E. coli K-12 and of lambda phage. 3. Hybrid colicin E1 DNAs having coslambda with molecular sizes corresponding to 28% of that of lambda phage DNA were packaged within lambda phage particles as trimers; hybrid DNAs with coslambda of 40 and 47% of the length of lambda phage DNA were packaged as dimers; and those with molecular sizes of 68% of that of lambda phage DNA were packaged mostly as monomers. These results demonstrated that two factors are essential for the packaging of DNAs within lambda phage particles; the presence of coslambda on the DNA molecule and an appropriate size of DNA.

Rolling circle replicative structure of bacteriophage lambda DNA in a recombination deficient system

Rolling-circle replicating structures which represent late stage lambda DNA replication can be detected among intracellular phage lambda DNA molecules under recombination deficient conditions as well as in wild-type infections. Furthermore, if initiation of lambda replication is delayed until the late stage of lambda infection, then nearly all replicating molecules are rolling-circle, even in the first round. Thus neither genetic recombination nor termination of a round of replication are required for generation of rolling-circle replicating molecules of lambda DNA.

Specialized transduction of colicin E1 DNA in Escherichia coli K-12

Genetic studies were made on E. coli K-12 TM96, which carries recombinant molecules constructed by in vitro combination of colicin E1 DNA and a DNA fragment of E. coli for guanine synthesis derived from transducing phage. The recombinant molecules existed as stable plasmids within the cell and contained genes for colicin E1 immunity and the guaA enzyme (xanthosine 5'-monophosphate aminase) together with a part of the lambda genome, R through J: (R-A-F-J)+. A block of the lambda genome, int through Q, was not detected in the recombinant molecule. Thus, this recombinant molecule was named ColEl-coslambda-guaA, and the specialized tranduction of the ColEl-coslambda-guA DNA into various E. coli K-12 cells by lambda phage was described. Lysates prepared by lytic infection of lambda phage onto TM96 or by induction of TM96(lambda) lysogens contained transducing particles which could transduce gua-deleted E. coli to stable guaA+ cells. These transductants were proved to have similar genetic properties as those of TM96. The frequency of transduction was not affected by the presence of an attachement site for lambda, prophage lambda, colicin E1 plasmids, or the recA property within gua-deleted recipient cells. Transducing particles were resistant to EDTA treatment and most of them had an average density of about 1.472. This value corresponds to that of lambda phage particles, which contain about 72% of the lenght of lambda DNA.