Breakage--reunion and copy choice mechanisms of recombination between short homologous sequences.

To study recombination between short homologous sequences in Escherichia coli we constructed plasmids composed of the pBR322 replicon, M13 replication origin and a recombination unit inserted within and inactivating a gene encoding chloramphenicol resistance. The unit was composed of short direct repeats (9, 18 or 27 bp) which flanked inverted repeats (0, 8 or 308 bp) and a gene encoding kanamycin resistance. Recombination between direct repeats restored a functional chloramphenicol resistance gene, and could be detected by a simple phenotype test. The plasmids replicated in a double-stranded form, using the pBR322 replicon, and generated single-stranded DNA when the M13 replication origin was activated. The frequency of chloramphenicol-resistant cells was low (10(-8)-10(-4] when no single-stranded DNA was synthesized but increased greatly (to 100%) after induction of single-stranded DNA synthesis. Recombination between 9 bp direct repeats entailed no transfer of DNA from parental to recombinant plasmids, whereas recombination between 18 or 27 bp repeats entailed massive transfer. The presence or length of inverted repeats did not alter the pattern of DNA transfer. From these results we propose that direct repeats of 9 bp recombine by a copy choice process, while those greater than or equal to 18 bp can recombine by a breakage-reunion process. Genome rearrangements detected in many organisms often occur by recombination between sequences less than 18 bp, which suggests that they may result from copy choice recombination.     

Plasmid inverted repeats provide for duplication or precise excision of DNA inserts

A plasmid containing inverted repeats is constructed in Bacillus subtilis. Insertion of DNA fragments into the plasmid inverted repeats results either in the precise excision of the insert or in its duplication in the opposite inverted repeat. These rearrangements are due to the presence of inverted repeats only. Two recombination events are possibly responsible for these phenomena. During the first step of the recombination two plasmid monomers form a dimer molecule. During the second step the intramolecular recombination between the direct repeats in the dimer structure leads to the formation of two rearranged plasmid monomers devoid of insertion or containing two DNA inserts. Proposed dimeric intermediate is unstable in B. subtilis. However, it is isolated from Escherichia coli recA, cells. Plasmids containing the inverted repeats can serve as a model to study plasmid recombination in B. subtilis cells.     

Orientation Dependence in Homologous Recombination

Homologous recombination was investigated in Escherichia coli with two plasmids, each carrying the homologous region (two defective neo genes, one with an amino-end deletion and the other with a carboxyl-end deletion) in either direct or inverted orientation. Recombination efficiency was measured in recBC sbcBC and recBC sbcA strains in three ways. First, we measured the frequency of cells carrying neo(+) recombinant plasmids in stationary phase. Recombination between direct repeats was much more frequent than between inverted repeats in the recBC sbcBC strain but was equally frequent in the two substrates in the recBC sbcA strain. Second, the fluctuation test was used to exclude bias by a rate difference between the recombinant and parental plasmids and led to the same conclusion. Third, direct selection for recombinants just after transformation with or without substrate double-strand breaks yielded essentially the same results. Double-strand breaks elevated recombination in both the strains and in both substrates. These results are consistant with our previous findings that the major route of recombination in recBC sbcBC strains generates only one recombinant DNA from two DNAs and in recBC sbcA strains generates two recombinant DNAs from two DNAs.     

Characterization of the hyperrecombination phenotype of the pol3-t mutation of Saccharomyces cerevisiae

The DNA polymerase delta (Pol3p/Cdc2p) allele pol3-t of Saccharomyces cerevisiae has previously been shown to increase the frequency of deletions between short repeats (several base pairs), between homologous DNA sequences separated by long inverted repeats, and between distant short repeats, increasing the frequency of genomic deletions. We found that the pol3-t mutation increased intrachromosomal recombination events between direct DNA repeats up to 36-fold and interchromosomal recombination 14-fold. The hyperrecombination phenotype of pol3-t was partially dependent on the Rad52p function but much more so on Rad1p. However, in the double-mutant rad1 Delta rad52 Delta, the pol3-t mutation still increased spontaneous intrachromosomal recombination frequencies, suggesting that a Rad1p Rad52p-independent single-strand annealing pathway is involved. UV and gamma-rays were less potent inducers of recombination in the pol3-t mutant, indicating that Pol3p is partly involved in DNA-damage-induced recombination. In contrast, while UV- and gamma-ray-induced intrachromosomal recombination was almost completely abolished in the rad52 or the rad1 rad52 mutant, there was still good induction in those mutants in the pol3-t background, indicating channeling of lesions into the above-mentioned Rad1p Rad52p-independent pathway. Finally, a heterozygous pol3-t/POL3 mutant also showed an increased frequency of deletions and MMS sensitivity at the restrictive temperature, indicating that even a heterozygous polymerase delta mutation might increase the frequency of genetic instability.     

The Detection of Inherent Homologous Recombination Between Repeat Sequences in H. pylori 26695 by the PCR-Based Method

Helicobacter pylori infects more than half of the world’s population, making it the most widespread infection of bacteria. It has high genetic diversity and has been considered as one of the most variable bacterial species. In the present study, a PCR-based method was used to detect the presence and the relative frequency of homologous recombination between repeat sequences (>500 bp) in H. pylori 26695. All the recombinant structures have been confirmed by sequencing. The inversion generated between inverted repeats showed distinct features from the recombination for duplication or deletion between direct repeats. Meanwhile, we gave the mathematic reasoning of a general formula for the calculation of relative recombination frequency and indicated the conditions for its application. This formula could be extensively applied to detect the frequency of homologous recombination, site-specific recombination, and other types of predictable recombination. Our results should be helpful for better understanding the genome evolution and adaptation of bacteria.     

Associations between inverted repeats and the structural evolution of bacterial genomes

The stability of the structure of bacterial genomes is challenged by recombination events. Since major rearrangements (i.e., inversions) are thought to frequently operate by homologous recombination between inverted repeats, we analyzed the presence and distribution of such repeats in bacterial genomes and their relation to the conservation of chromosomal structure. First, we show that there is a strong under-representation of inverted repeats, relative to direct repeats, in most chromosomes, especially among the ones regarded as most stable. Second, we show that the avoidance of repeats is frequently associated with the stability of the genomes. Closely related genomes reported to differ in terms of stability are also found to differ in the number of inverted repeats. Third, when using replication strand bias as a proxy for genome stability, we find a significant negative correlation between this strand bias and the abundance of inverted repeats. Fourth, when measuring the recombining potential of inverted repeats and their eventual impact on different features of the chromosomal structure, we observe a tendency of repeats to be located in the chromosome in such a way that rearrangements produce a smaller strand switch and smaller asymmetries than expected by chance. Finally, we discuss the limitations of our analysis and the influence of factors such as the nature of repeats, e.g., transposases, or the differences in the recombination machinery among bacteria. These results shed light on the challenges imposed on the genome structure by the presence of inverted repeats.     

Formation of extrachromosomal circular amplicons with direct or inverted duplications in drug-resistant Leishmania tarentolae.

Selection for methotrexate resistance in Leishmania spp. is often associated with amplification of the H locus short-chain dehydrogenase-reductase gene ptr1 as part of extrachromosomal elements. Extensive sequences are always coamplified and often contain inverted duplications, most likely formed by the annealing of inverted repeats present at the H locus. By gene targeting mediated by homologous recombination, several repeated sequences were introduced in the vicinity of ptr1. Selection for methotrexate resistance in these transfectants led to ptr1 amplification as part of small circles with direct or inverted duplications whether the integrated sequences consisted of direct or inverted repeats. Hence, for a region to he amplified in L. tarentolae during drug selection, a drug resistance gene is required and must be flanked by (any) homologous repeated sequences. The distance between these repeats and their orientation will determine the length of the amplicon and whether it contains direct or inverted duplications.     

Structural plasmid instability in Bacillus subtilis: Effect of direct and inverted repeats

Summary Using precise excision as a model system, we have quantified the effect of direct repeats, inverted repeats and the size of the spacer between the repeats in the process of deletion formation in Bacillus subtilis. Both in the presence and absence of inverted repeats, the frequency of precise excision was strongly dependent on the direct repeat length. By increasing the direct repeat length from 9 bp to 18 and 27 bp, the precise excision frequency was raised by 3 and 4 orders of magnitude, respectively. In addition, irrespective of the direct repeat length, the presence of flanking inverted repeats enhanced the excision frequency by 3 orders of magnitude. Varying the inverted repeat length and the spacer size over a wide range did not significantly affect the excision frequencies. These results fit well into a model for deletion formation by slipped mispairing during replication of single-stranded plasmid DNA.     

Ac insertion site affects the frequency of transposon-induced homologous recombination at the maize p1 locus

The maize p1 gene regulates the production of a red pigment in the kernel pericarp, cob, and other maize floral tissues. Insertions of the transposable element Ac can induce recombination between two highly homologous 5.2-kb direct repeat sequences that flank the p1 gene-coding region. Here, we tested the effects of the Ac insertion site and orientation on the induction of recombination at the p1 locus. A collection of unique p1 gene alleles was used, which carry Ac insertions at different sites in and near the p1 locus, outside of the direct repeats, within the direct repeat sequences, and between the direct repeats, in both orientations. Recombination was scored by the numbers of colorless pericarp sectors (somatic frequency) and heritable mutations (germinal frequency). In both the somatic and germinal tests, the frequency of homologous recombination is significantly higher when Ac is inserted between the direct repeats than when Ac is inserted either within or outside the repeats. In contrast, Ac orientation had no significant effect on recombination frequency. We discuss these results in terms of the possible mechanisms of transposon-induced recombination.     

Structure and evolution of a family of interspersed repetitive DNA sequences inCaenorhabditis elegans

Summary The structure of three members of a repetitive DNA family from the genome of the nematodeCaenorhabditis elegans has been studied. The three repetitive elements have a similar unitary structure consisting of two 451-bp sequences in inverted orientation separated by 491 bp, 1.5 kb, and 2.5 kb, respectively. The 491-bp sequence separating the inverted 451-bp sequences of the shortest element is found adjacent to one of the repeats in the other two elements as well. The combination of the three sequences we define as the basic repetitive unit. Comparison of the nucleotide sequences of the three elements has allowed the identification of the one most closely resembling the primordial repetitive element. Additionally, a process of co-evolution is evident that results in the introduction of identical sequence changes into both copies of the inverted sequence within a single unit. Possible mechanisms are discussed for the homogenization of these sequences. A direct test of one possible homogenization mechanism, namely homologous recombination between the inverted sequences accompanied by gene conversion, shows that recombination between the inverted repeats does not occur at high frequency.     

The Influence of Primary and Secondary DNA Structure in Deletion and Duplication between Direct Repeats in Escherichia Coli

We describe a system to measure the frequency of both deletions and duplications between direct repeats. Short 17- and 18-bp palindromic and nonpalindromic DNA sequences were cloned into the EcoRI site within the chloramphenicol acetyltransferase gene of plasmids pBR325 and pJT7. This creates an insert between direct repeated EcoRI sites and results in a chloramphenicol-sensitive phenotype. Selection for chloramphenicol resistance was utilized to select chloramphenicol resistant revertants that included those with precise deletion of the insert from plasmid pBR325 and duplication of the insert in plasmid pJT7. The frequency of deletion or duplication varied more than 500-fold depending on the sequence of the short sequence inserted into the EcoRI site. For the nonpalindromic inserts, multiple internal direct repeats and the length of the direct repeats appear to influence the frequency of deletion. Certain palindromic DNA sequences with the potential to form DNA hairpin structures that might stabilize the misalignment of direct repeats had a high frequency of deletion. Other DNA sequences with the potential to form structures that might destabilize misalignment of direct repeats had a very low frequency of deletion. Duplication mutations occurred at the highest frequency when the DNA between the direct repeats contained no direct or inverted repeats. The presence of inverted repeats dramatically reduced the frequency of duplications. The results support the slippage-misalignment model, suggesting that misalignment occurring during DNA replication leads to deletion and duplication mutations. The results also support the idea that the formation of DNA secondary structures during DNA replication can facilitate and direct specific mutagenic events.     

On the Deletion of Inverted Repeated DNA in Escherichia Coli: Effects of Length, Thermal Stability, and Cruciform Formation in Vivo

We have studied the deletion of inverted repeats cloned into the EcoRI site within the CAT gene of plasmid pBR325. A cloned inverted repeat constitutes a palindrome that includes both EcoRI sites flanking the insert. In addition, the two EcoRI sites represent direct repeats flanking a region of palindromic symmetry. A current model for deletion between direct repeats involves the formation of DNA secondary structure which may stabilize the misalignment between the direct repeats during DNA replication. Our results are consistent with this model. We have analyzed deletion frequencies for several series of inverted repeats, ranging from 42 to 106 bp, that were designed to form cruciforms at low temperatures and at low superhelical densities. We demonstrate that length, thermal stability of base pairing in the hairpin stem, and ease of cruciform formation affect the frequency of deletion. In general, longer palindromes are less stable than shorter ones. The deletion frequency may be dependent on the thermal stability of base pairing involving approximately 16-20 bp from the base of the hairpin stem. The formation of cruciforms in vivo leads to a significant increase in the deletion frequency. A kinetic model is presented to describe the relationship between the physical-chemical properties of DNA structure and the deletion of inverted repeats in living cells.     

The effect of the length of direct repeats and the presence of palindromes on deletion between directly repeated DNA sequences in bacteriophage T7.

The frequency of genetic deletion between directly repeated DNA sequences in bacteriophage T7 was measured as a function of the length of the direct repeat. The non-essential ligase gene (gene 1.3) of bacteriophage T7 was interrupted with pieces of synthetic DNA bracketed by direct repeats of various lengths. Deletion of these 76 bp long inserts was too low to be measured when the direct repeats were less than 6 bp long. However, the frequency of deletion of inserts with longer direct repeats increased exponentially as the length of the repeats increased from 8 to 20 bp. When inverted repeats (palindromes) were designed in the midst of the insert there was essentially no increase in deletion frequency between 10 bp direct repeats. But, the same palindromic sequences increased the deletion frequency between 5 bp direct repeats by at least two orders of magnitude. Thus, in this system homology at the endpoints is a more important determinant of deletion frequency than is the presence of palindromes between the direct repeats.     

The DNA sequences of T-DNA junctions suggest that complex T-DNA loci are formed by a recombination process resembling T-DNA integration

After Agrobacterium-mediated plant transformation, multiple T-DNAs frequently integrate at the same position in the plant genome, resulting in the formation of inverted and direct repeats. Because these inverted repeats cannot be amplified and analyzed by PCR, Arabidopsis root cells were co-transformed with two different T-DNAs with distinct sequences adjacent to the T-DNA borders. Nine direct or inverted T-DNA border junctions were analyzed at the sequence level. Precise end-to-end fusions were found between two right border ends, whereas imprecise fusions and filler DNA were present in T-DNA linkages containing a left border end. The results suggest that end-to-end ligation of double-stranded T-DNAs occurs especially between right T-DNA ends and that illegitimate recombination on the basis of microhomology, deletions, repair activities and insertions of filler DNA is involved in the formation of left border T-DNA junctions. Therefore, a similar illegitimate recombination mechanism is proposed that is involved in the formation of complex T-DNA inserts as well as in the integration of the T-DNA in the plant genome.     

The Effects of Insertions on Mammalian Intrachromosomal Recombination

We examined the effects of insertion mutations on intrachromosomal recombination. A series of mouse L cell lines carrying mutant herpes simplex virus thymidine kinase (tk) heteroalleles was generated; these lines differed in the nature of their insertion mutations. In direct repeat lines with different large insertions in each gene, there was a 20-fold drop in gene conversion rate and only a five-fold drop in crossover rate relative to the analogous rates in lines with small insertions in each gene. Surprisingly, in direct repeat lines carrying the same large insertion in each gene, there was a larger drop in both types of recombination. When intrachromosomal recombination between inverted repeat tk genes with different large insertions was examined, we found that the rate of gene conversion dropped five-fold relative to small insertions, while the rate of crossing over was unaffected. The differential effects on conversion and crossing over imply that gene conversion is more sensitive to insertion mutation size. Finally, the fraction of gene conversions associated with a crossover increased from 2% for inverted repeats with small insertions to 18% for inverted repeats with large insertions. One interpretation of this finding is that during intrachromosomal recombination in mouse cells long conversion tracts are more often associated with crossing over.     

Analysis of DNA repeats in bacterial plasmids reveals the potential for recurrent instability events

Structural instability has been frequently observed in natural plasmids and vectors used for protein expression or DNA vaccine development. However, there is a lack of information concerning hotspot mapping, namely, DNA repeats or sequences identical to the host genome. This led us to evaluate the abundance and distribution of direct, inverted, and tandem repeats with high recombination potential in 36 natural plasmids from ten bacterial genera, as well as in several widely used bacterial and mammalian expression vectors. In natural plasmids, we observed an overrepresentation of close direct repeats in comparison to inverted ones and a preferential location of repeats with high recombination potential in intergenic regions, suggesting a highly plastic and dynamic behavior. In plasmid vectors, we found a high density of repeats within eukaryotic promoters and non-coding sequences. As a result of this in silico analysis, we detected a spontaneous recombination between two 21-bp direct repeats present in the human cytomegalovirus early enhancer/promoter (huCMV EEP) of the pCIneo plasmid. This finding is of particular importance, as the huCMV EEP is one of the most frequently used regulatory elements in plasmid vectors. Because pDNA integration into host gDNA can have adverse consequences in terms of plasmid processing and host safety, we also mapped several regions with high probability to mediate integration into the Escherichia coli or human genomes. Like repeated regions, some of these were located in non-coding regions of the plasmids, thus being preferential targets to be removed.     

Direct and Inverted Repeats Elicit Genetic Instability by Both Exploiting and Eluding DNA Double-Strand Break Repair Systems in Mycobacteria

Repetitive DNA sequences with the potential to form alternative DNA conformations, such as slipped structures and cruciforms, can induce genetic instability by promoting replication errors and by serving as a substrate for DNA repair proteins, which may lead to DNA double-strand breaks (DSBs). However, the contribution of each of the DSB repair pathways, homologous recombination (HR), non-homologous end-joining (NHEJ) and single-strand annealing (SSA), to this sort of genetic instability is not fully understood. Herein, we assessed the genome-wide distribution of repetitive DNA sequences in the Mycobacterium smegmatis, Mycobacterium tuberculosis and Escherichia coli genomes, and determined the types and frequencies of genetic instability induced by direct and inverted repeats, both in the presence and in the absence of HR, NHEJ, and SSA. All three genomes are strongly enriched in direct repeats and modestly enriched in inverted repeats. When using chromosomally integrated constructs in M. smegmatis, direct repeats induced the perfect deletion of their intervening sequences ∼1,000-fold above background. Absence of HR further enhanced these perfect deletions, whereas absence of NHEJ or SSA had no influence, suggesting compromised replication fidelity. In contrast, inverted repeats induced perfect deletions only in the absence of SSA. Both direct and inverted repeats stimulated excision of the constructs from the attB integration sites independently of HR, NHEJ, or SSA. With episomal constructs, direct and inverted repeats triggered DNA instability by activating nucleolytic activity, and absence of the DSB repair pathways (in the order NHEJ>HR>SSA) exacerbated this instability. Thus, direct and inverted repeats may elicit genetic instability in mycobacteria by 1) directly interfering with replication fidelity, 2) stimulating the three main DSB repair pathways, and 3) enticing L5 site-specific recombination.     

Deletion mutagenesis independent of recombination in bacteriophage T7.

Deletion between directly repeated DNA sequences in bacteriophage T7-infected Escherichia coli was examined. The phage ligase gene was interrupted by insertion of synthetic DNA designed so that the inserts were bracketed by 10-bp direct repeats. Deletion between the direct repeats eliminated the insert and restored the ability of the phage to make its own ligase. The deletion frequency of inserts of 85 bp or less was of the order of 10(-6) deletions per replication. The deletion frequency dropped sharply in the range between 85 and 94 bp and then decreased at a much lower rate over the range from 94 to 900 bp. To see whether a deletion was predominantly caused by intermolecular recombination between the leftmost direct repeat on one chromosome and the rightmost direct repeat on a distinct chromosome, genetic markers were introduced to the left and right of the insert in the ligase gene. Short deletions of 29 bp and longer deletions of approximately 350 bp were examined in this way. Phage which underwent deletion between the direct repeats had the same frequency of recombination between the left and right flanking markers as was found in controls in which no deletion events took place. These data argue against intermolecular recombination between direct repeats as a major factor in deletion in T7-infected E. coli.     

Altered directionality in the Cre-LoxP site-specific recombination pathway

The site-specific recombinase Cre must employ control mechanisms to impose directionality on recombination. When two recombination sites (locus of crossing over in phage P1, loxP) are placed as direct repeats on the same DNA molecule, collision between loxP-bound Cre dimers leads to excision of intervening DNA. If two sites are placed as inverted repeats, the intervening segment is flipped around. Cre catalyzes these reactions in the absence of protein co-factors. Current models suggest that directionality is controlled at two steps in the recombination pathway: the juxtaposition of loxP sites and the single-strand-transfer reactions within the synaptic complex. Here, we show that in Escherichia coli strain 294-Cre, directionality for recombination is altered when the expression of Cre is increased. This leads to deletion instead of inversion on substrates carrying two loxP sites as inverted repeats. The nucleotide sequence composition of loxP sites remaining in aberrant products indicates that site alignment and/or DNA strand transfer in the in vivo Cre-loxP recombination pathway are not always tightly controlled.