The variation in frequency with which markers are transduced by phage P1 is primarily a result of discrimination during recombination

Discovery, purification, and characterization of a temperate transducing bacteriophage for Bordetella avium

We discovered and characterized a temperate transducing bacteriophage (Ba1) for the avian respiratory pathogen Bordetella avium. Ba1 was initially identified along with one other phage (Ba2) following screening of four strains of B. avium for lysogeny. Of the two phage, only Ba1 showed the ability to transduce via an allelic replacement mechanism and was studied further. With regard to host range, Ba1 grew on six of nine clinical isolates of B. avium but failed to grow on any tested strains of Bordetella bronchiseptica, Bordetella hinzii, Bordetella pertussis, or Bordetella parapertussis. Ba1 was purified by CsCl gradient centrifugation and was found to have an icosahedral head that contained a linear genome of approximately 46.5 kb (contour length) of double-stranded DNA and a contractile, sheathed tail. Ba1 readily lysogenized our laboratory B. avium strain (197N), and the prophage state was stable for at least 25 generations in the absence of external infection. DNA hybridization studies indicated the prophage was integrated at a preferred site on both the host and phage replicons. Ba1 transduced five distinctly different insertion mutations, suggesting that transduction was generalized. Transduction frequencies ranged from approximately 2 x 10(-7) to 1 x 10(-8) transductants/PFU depending upon the marker being transduced. UV irradiation of transducing lysates markedly improved transduction frequency and reduced the number of transductants that were lysogenized during the transduction process. Ba1 may prove to be a useful genetic tool for studying B. avium virulence factors.

Generalized transduction in the potato blackleg pathogen Erwinia carotovora subsp. atroseptica by bacteriophage M1

Summary: Using enrichment methods, a new bacteriophage (M1) was isolated, which is capable of generalized transduction in Erwinia carotovora subsp. atroseptica (Eca) strain SCRI1043. M1 is probably a virulent phage and contains double-stranded DNA of approximately 43 kb. Transduction frequencies for a number of chromosomal markers and plasmid pHCP2 were established, and conditions for transduction optimized. UV irradiation of the lysates prior to transduction enhanced the transduction frequency. M1 infected over 25% of Eca strains tested and so may be useful both for the genetic analysis of a number of Eca isolates and for the transductional transfer of selectable markers between strains.

Proteins encoded by the Escherichia coli replication terminus region

The replication terminus region (31 to 35 min) of the Escherichia coli chromosome contains very few mapped genes (two per min) compared with the remainder of the chromosome, and much of the DNA appears dispensable. In order to determine whether, despite this, the terminus region consists of protein-coding sequences, we cloned 44 kb (1 min) of terminus region DNA (that surrounding trg at 31.4 min) and examined its ability to catalyze protein synthesis in vitro or in minicells. We were able to account for more than half the coding capacity of the cloned DNA with proteins synthesized in these systems, indicating that the sparsity of mapped genes in the terminus region does not result from a lack of identifiable coding sequences. We can therefore conclude that the terminus region is composed mainly of expressable, albeit inessential, protein-encoding genes.

Packaging of transducing DNA by bacteriophage P1

P1 transduces bacterial chromosomal markers with widely differing frequencies. We use quantitative Southern hybridisations here to show that, despite this, most markers are packaged at similar levels. Exceptions are a group of markers near 2 min and another at 90 min which seem to be packaged at levels two- to threefold higher. We thus conclude that certain marker frequency variations in transduction can be explained by differences in packaging level, but that most cannot. The limited range in packaging levels suggests that P1 can initiate the packaging of chromosomal DNA from many sites. This idea is supported by our failure to find any chromosomal sequences with homology to the phage pac site and by the occurrence of hybridising bands which seem to suggest sequential packaging from a large number of specific sites. We eliminate the possibility that chromosomal DNA packaging is the result of endonucleolytic cutting by the P1 res enzyme.

Reduction of marker discrimination in transductional recombination

The recovery of phage P1 mediated transductants varies with the marker selected in a manner which cannot be fully accounted for by dosage differences in the donor gene population. This variation in transduction frequency is due primarily to recombinational discrimination in the recipient cell. We show here that increasing the intracellular level of recA protein, which might be expected to increase the contribution of recF mediated events to recombinant formation, decreases this discrimination slightly, and that replacing recBC mediated recombination by a recF dependent process, augmented by an additional, as yet uncharacterized mutation, dramatically reduces recombinational discrimination. We conclude that although recBC mediated transductional recombination is selective, recombination which relies on recF need not be so. We also show that UV-damaged DNA can be successfully recombined in the absence of the recB product (even in sbcB+ cells) and that eliminating exonuclease I (the sbcB product) facilitates the recombination of heavily irradiated DNA.

Physical and genetical analysis of bacteriophage T4 generalized transduction

This report describes a comparison of the efficiency of transduction of genes in E. coli by the generalized transducing bacteriophages T4GT7 and P1CM. Both phages are capable of transducing many genetic markers in E. coli although the frequency of transduction for particular genes varies over a wide range. The frequency of transduction for most genes depends on which transducing phage is used as well as on the donor and recipient bacterial strains. Analysis of T4GT7 phage lysates by cesium chloride density gradient centrifugation shows that transducing phage particles contain primarily bacterial DNA and carry little, if any, phage DNA. In this regard transducing phages P1CM and T4GT7 are similar; both phages package either bacterial or phage DNA but not both DNAs into the same particle.

Escherichia coli minichromosomes containing the pSC101 partitioning locus are stably inherited

The par region of pSC101, required in cis to promote its stable inheritance, was joined, in combination with the tetr determinant of pBR325, to large and small minichromosomes. These hybrid minichromosomes were examined for stability and found to be no more stable than their parent minichromosomes. Indeed, one recombinant plasmid, pEH21, showed reduced stability, which was not attributable to a reduced copy number. Neither pEH21 nor pEH22, a plasmid composed of the same DNA arranged differently, was stabilized by the presence of a Par+ pSC101 derived replicon in the same cell. We conclude that the par region of pSC101 does not stabilize minichromosomes.

Genetic and biochemical analyses of Escherichia coli mutants altered in the temperature-dependent regulation of membrane lipid composition

We have previously studied two mutants of Escherichia coli altered in the regulation of membrane lipid composition by temperature. One class (represented by the fabFl allele) fails to regulate upon temperature shift and is defective in cis-vaccenic acid synthesis owing to the lack of the fatty acid elongation enzyme beta-ketoacyl-acyl carrier protein synthase II(EC 2.3.1.41). A second class of mutant, given the phenotypic designation Vtr, overproduces cis-vaccenic acid at all temperatures and hence is altered in temperature regulation. In this paper we report evidence for the following conclusions. (i) The Vtr and fabFl mutations show very tight genetic linkage. (ii) The Vtr lesion is allelic to the fabFl mutation since the presence of the fabFl mutation in merodiploid strains carrying the Vtr or fabF(+) alleles results in fatty acid compositions intermediate between those of the two monoploid strains. Merodiploids carrying both the fabF(+) and Vtr alleles likewise show an intermediate composition. These results indicate intra-allelic complementation. (iii) The two E. coli proteins recently discovered by Rock (J. Bacteriol. 152:1298-1300, 1982) that form mixed disulfide cross-links to acyl carrier protein are directly demonstrated to be beta-ketoacyl-acyl carrier protein synthases I and II. (iv) The fabFl strains produce a synthase II band of altered electrophoretic mobility, indicating that the fabF locus is the structural gene for synthase II. (v) The synthase II of Vtr strains is abnormally sensitive to cerulenin, an antibiotic that specifically inhibits synthases I and II. This increased sensitivity is readily demonstrated in vivo, but in vitro we failed to detect an increased sensitivity of the Vtr synthase II to cerulenin, nor have we detected any other kinetic or structural alteration in the enzyme. We interpret these results in terms of specific interactions of synthase II with other cellular components which occur in vivo but are not duplicated in vitro.

Recombination-induced suppression of cell division following P1-mediated generalized transduction in Klebsiella aerogenes

Klebsiella aerogenes recombinants resulting from bacteriophage P1-mediated generalized transduction failed to increase in number for approximately six generations after transduction. Nevertheless these recombinants continued to grow and became sensitive to penicillin after a transient resistance, suggesting that the cells were growing as long, non-dividing filaments. When filamentous cells were isolated from transduced cultures by gradient centrifugation, recombinants were 1000-fold more frequent among the filaments than among the normal-sized cells. The suppression of cell-division lasted for six generations whether markers near the origin (gln, ilv) or terminus (his, trp) of chromosome replication were used, despite a 50-fold difference in transduction frequencies for these markers. The suppression of cell division was a host response to recombination rather than to P1 invasion since cells lysogenized by P1 in these same experiments showed only a short (two generation) suppression of cell division. We speculate that the suppression of cell-division is an SOS response triggered by the degraded DNA not incorporated in the final recombinant. We demonstrate that both the filamentation and the transient penicillin resistance of recombinant cells can be exploited to enrich greatly for recombinants, raising transduction frequencies to as high as 10(-3).

Mapping nonselectable genes of Escherichia coli by using transposon Tn10: location of a gene affecting pyruvate oxidase

Mutants of Escherichia coli K-12 deficient in pyruvate oxidase were isolated by screening for the production of 14CO2 from [1-14C]pyruvate by the method of Tabor et al. (J. Bacteriol. 128:485-486, 1976). One of these lesions (designated poxA) decreased the pyruvate oxidase activity to 10 to 15% of the normal level but grew well. To map this nonselectable mutation, we isolated strains having transposon Tn10 inserted into the chromosome close to the poxA locus and mapped the transposon. These insertions were isolated by the following procedure: (i) pools of Tn10 insertions into the chromosomes of two different Hfr strains were prepared by transposition from a lambda::Tn10 vector; (ii) these Tn10-carrying strains were then mated with a poxA recipient strain, and tetracycline-resistant (Tetr) recombinants were selected; (iii) the Tetr recombinants were then screened for 14CO2 production from [1-14C]pyruvate. This method was shown to give a greater than 40-fold enrichment of insertions of Tn10 near the poxA gene as compared with transduction. Calculations indicate that a similar enrichment should be expected for other genes. The enrichment is due to the much greater map interval over which strong linkage between selected and unselected markers is found in conjugational crosses as compared with transductional crosses. The use of Hfr conjugative transfer allows isolation of transposon insertions closely linked to a nonselectable gene by scoring hundreds rather than thousands of colonies. Using a Tn10 insertion greater than 98% cotransduced with the poxA locus, we mapped the poxA gene on the E. coli genetic map. The poxA locus is located at 94 min, close to the psd locus. The clockwise gene order is ampA, poxA, psd, purA. The poxA mutation is recessive and appears to be a regulatory gene.

Construction of a genetic map for Caulobacter crescentus

RP4-mediated conjugation has been used to transfer large fragments of chromosomal material in Caulobacter crescentus. In this system, conjugation proceeds from multiple origins, and haploid recombinants are recovered at frequencies of 10(-6) and 10(-7) per donor cell. The data from five-factor crosses were subjected to computer-assisted crossover analyses as a rapid method to determine marker order. Using this information and data from additional two- and three-factor crosses mediated by RP4 or the generalized transducing bacteriophage phi Cr30, we constructed the first genetic map for C. crescentus.