TEMPERATURE-SENSITIVE MUTANTS OF COLIPHAGE LAMBDA

Mutations in bacteriophage lambda affecting host cell lysis

Nine temperature-sensitive and defective mutants of phage lambda deficient in lysis of the host cell have been analyzed in respect to location of the site of mutation and phenotypic properties. Seven of the isolates are mutant in cistron R and produce either a thermolabile endolysin or no detectable enzyme. The remaining two mutants, one temperature-sensitive and one defective, produce high levels of apparently normal enzyme. These mutants denote the involvement of at least one phage cistron other than R in the lytic process.

Gene products encoded in the ninR region of phage lambda participate in Red-mediated recombination

BACKGROUND

The ninR region of phage lambda contains two recombination genes, orf (ninB) and rap (ninG), that were previously shown to have roles when the RecF and RecBCD recombination pathways of E. coli, respectively, operate on phage lambda.

RESULTS

When lambda DNA replication is blocked, recombination is focused at the termini of the virion chromosome. Deletion of the ninR region of lambda decreases the sharpness of the focusing without diminishing the overall rate of recombination. The phenotype is accounted for in large part by the deletion of rap and of orf. Mutation of the recJ gene of the host partially suppresses the Rap- phenotype.

CONCLUSION

ninR functions Orf and Rap participate in Red recombination, the primary pathway operating when wild-type lambda grows lytically in rec+ cells. The ability of recJ mutation to suppress the Rap- phenotype indicates that RecJ exonuclease can participate in Red-mediated recombination, at least in the absence of Rap function. A model is presented for Red-mediated RecA-dependent recombination that includes these newly identified participants.

In vivo packaging of bacteriophage lambda monomeric chromosomes

There is an apparent paradox between the reported requirements for lambda DNA packaging in vivo and in vitro. In vivo, DNA concatemers are required for packaging. On the other hand, in vitro, packaging extracts can encapsidate either linear or circular monomeric lambda DNA. Perhaps cellular nucleases restrict the in vivo ability of monomers to package by degrading a free double chain end present as an intermediate in the packaging reaction. Consistent with this hypothesis, enhanced packaging of monomers was found in an ExoV- host. No additional enhancement was noted in a host also mutant for sbcB and sbcC. We isolated a mutant phage for which in vivo packaging of monomeric lambda chromosomes is increased about 10(3)-fold. The responsible mutation (plm1 for packages lambda monomers) was mapped to cro, sequenced, and found to cause a change from Ala29 to Ser in the alpha3 helix of Cro's DNA binding domain. Density transfer experiments showed that packaging of both plm1 and wild-type lambda was aided by allowing some DNA synthesis. However, the packaged chromosomes had not themselves undergone a full round of replication and therefore were not part of a canonical concatemer made by replication. Other tests showed that packaged phage had not been part of concatemers made by recombination or by annealing at cos. Our results with wild-type lambda also favor models in which two cos sites are needed for packaging, but these sites need not be in cis. In lambda plm1, replication intermediates may serve as substrates for encapsidation.

The carboxy-terminal 14 amino acids of phage lambda N protein are dispensable for transcription antitermination

The analogous N proteins encoded by lambdoid bacteriophages lambda, 21, and 22 are very different in amino acid sequence, except at their carboxy-terminal ends. Since N lambda remains functional despite the deletion of most of its terminal region of homology to N21, that region of homology cannot represent a region of conserved function.

Double-chain-cut sites are recombination hotspots in the Red pathway of phage lambda

The Red recombination pathway of phage lambda is shown to target recombination to double-chain ends of DNA. A double-chain cut, delivered in vivo to only one of two parents participating in a lambda lytic cross by a type II restriction endonuclease, increases the proportion of crossing over in the interval containing the cut compared with other intervals. The stimulating effect of a cut is evident whether replication is inhibited or permitted. Cut stimulation can move away from the initial cut-site, presumably by double-chain degradation. Movement of the stimulating effect of a cut is dependent on the Escherichia coli gene recA when the cross is carried out under conditions that inhibit phage replication. When replication is permitted, all aspects of cut-stimulated recombination are independent of recA. Evidence is presented to show that the reaction that is stimulated by cutting is often non-reciprocal at the molecular level.

In phage lambda, cos is a recombinator in the red pathway

Among lambda particles carrying chromosomes that have failed to replicate during a lytic cycle cross there is a high frequency of Red-mediated recombination near the right-hand end. Earlier work has shown that this recombination is dependent on cos (cohesive end site), the packaging origin of lambda. In contrast to the prediction of the break-copy model proposed earlier, we find a high recombination rate near cos even when only one of the two participating parents has a functional cos at that locus. The exchange is accompanied by loss of the stimulating cos in the recombination product, irrespective of the marker configurations: a+b+cos- rather than a+b+cos+ is produced in the cross a+b-cos- x a-b+cos+ as well as in the cross a+b-cos+ x a-b+cos-. Further analyses of these and earlier data allow the formulation of a detailed model for cos-stimulated, Red-mediated genetic exchange. In this model, cos stimulates exchange by virtue of being a double-strand cut site. The model has several features like that proposed for yeast. This role of cos in the Red pathway contrasts with the role of cos in the RecBC pathway, in which cos serves as an entry site for a recombinase that stimulates exchanges far from cos.

Activation of Chi, a recombinator, by the action of an endonuclease at a distant site

Chi is an Escherichia coli recombinator specific to the recBC pathway. In phage lambda, its activity is dependent on its orientation with respect to the orientation of the chromosome packaging origin, cos [Kobayashi, I., Murialdo, H., Crasemann, J.M., Stahl, M. M. & Stahl, F. W. (1982) Proc. Natl. Acad. Sci. USA 79, 5981-5985]. Chi in an inactive state is activated by the in vivo action of a restriction endonuclease at a distant site. This result supports a model for Chi activation in which a recombination machine enters a DNA duplex at an end and travels until it encounters a Chi properly oriented with respect to the direction of enzyme movement. Events are then initiated that lead to exchange in the neighborhood of Chi.

Chi activity during transduction-associated recombination

Chi is a genetic element that stimulates phage lambda recombination by the Escherichia coli recBC pathway during lytic infection [Stahl, F. W. (1979) Annu. Rev. Genet. 13, 7--24]. Herein we show that chi in lambda prophage influences exchange distribution in P1 phage-mediated transduction and in conjugation. This demonstration encourages the view that chi may influence genetic exchange in E. coli in the total absence of lambda.

Isolation of suppressor sensitive mutants in the Ai gene of phage lambda

Previous experiments have shown that mutations in the Ai gene can suppress the growth defect of lambda N- phages. Many temperature resistant derivatives of phage lambdatsN9 have been isolated and among these 5 have been found which are Ai- and have an amber suppressible behaviour. These mutants can help in defining the role of the Ai gene in phage lambda development.

The role of Chi mutations in the Spi- phenotype of phage lambda: lack of evidence for a gene delta

Lambda red- gam- makes a small plaque on P2 lysogens (the partial Spi- phenotype). It has been proposed that inactivation of an additional gene delta, mapping in the recombination region, makes the plaque bigger (the full Spi- phenotype) (Zissler et al., 1917b). The present paper demonstrates that the Chi mutation in lambda (Stahl et al., 1975) confers upon red- gam- phage the full Spi- phenotype and that the deletion of the region of the chromosome attributed to delta does not. It appears unnecessary to invoke a gene delta in the Spi- phenotype.

Control of gene expression in bacteriophage lambda: suppression of N mutants by mutations of the antirepressor

The lysogenization and induction properties of phages lambdasusN7CI857Ai7 and lambdasusN53cro27 are described. Both phages, at 32 degrees kill little, but show only a moderate frequency of lysogenization whether an amber suppressor is present or absent in the host bacterium. In the latter case, lysogens for lambdasusN7CI857Ai7 or lambdasusN53CI857cro27 can exist in two different regulatory states, here called P r- and Pr+. The Pr+ phase is characterized by phage release and cell death at 40 degrees; conversely, cells in the Pr- phase are similarly killed but release no or very little phage. Pr- is the phase usually obtained at lysogenization. Each phase may be transmitted at 32 degrees for an unlimited number of generations, however, shifts to the opposite phase take place from time to time with a low probability. Two previously described antirepressor defective mutants. Ai7 and cro27, were found to suppress specifically the growth defect caused by an amber mutation in gene N. This suppression is observed in non-suppressing hosts at 40 or 42 degrees. Apparent revertants of N- mutants were shown to be often (80%) caused by a second mutation, in the Ai gene (also called tof, cro and fed). All the revertants so far examined appeared to be recessive. Lambda phages bearing a double amber mutation in gene N did not acquire full N independence by the acquisiton of an Ai mutation; this could be achieved, however, in the presence of a CII mutation. The above findings are discussed in terms of a direct interaction between the N, Ai and CII products.

The role of the N gene of phage lambda in the synthesis of two phage-specific proteins

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Temperature-sensitive steps in the biosynthesis of Venezuelan equine encephalitis virus

In contrast to Eastern equine encephalitis virus, the replication of Venezuelan equine encephalitis (VEE) virus was strongly inhibited at 44 C in chick embryo cells. The inhibited steps were analyzed by shifting the incubating temperatures up or down, and by determining during the shifts the rate and extent of infectious ribonucleic acid (RNA) synthesis, intact virus synthesis, and formation of complement-fixing antigen or of antigen detectable by a direct fluorescent-antibody technique. The inhibition appeared to be due to two temperature-sensitive steps involved in the synthesis of VEE virus in chick embryo cells. The first step of inhibition at 44 C occurred early in virus replication and could be completely reversed simply by transferring cultures to 37 C. The inhibition appeared to take place at some point between the time when the virus entered the cell and was uncoated and the beginning of viral RNA synthesis. The second temperature-sensitive step in VEE virus synthesis was irreversible; it occurred at a point after the synthesis of viral RNA, and before the formation of virus protein measured as complement-fixing antigen or as antigen that could be stained with fluorescent antibody.

The structure and function of the DNA from bacteriophage lambda

The position and orientation of genes in lambda and lambda dg DNA are described. The position of six genes located in the right half of isolated lambda DNA was found to be -(N, i(lambda))--O-P---Q-R-(right end of DNA), which is their order on the genetic map of the vegetative phage. The order of the three genes of the galactose operon (k, t, and e) located in the left half of lambda dg DNA was found to be (left end of DNA)----k-t-e-, consistent with Campbell's model (5) for the formation of this variant. Gene orientation, defined as the direction of transcription along the DNA, is inferred to be from right to left for the galactose operon in lambda dg DNA. The strand of lambda DNA which functions as template in transcription of N, an "early" gene required for normal replication of lambda DNA, was determined as a first step in ascertaining the orientation of this gene. The method includes isolation of each strand, formation of each of two heteroduplex molecules consisting of one strand from wild-type and one from an N mutant) and comparison of their N activities. The second step, which consists of ascertaining the 5'-to-3' direction of each strand, is discussed, as is a determination of the orientation of gene R.