Evidence for two mechanisms of palindrome-stimulated deletion in Escherichia coli: single-strand annealing and replication slipped mispairing

A series of bidirectional tetracycline-inducible promoters provides coordinated protein expression

Coordinated control of protein expression is highly desirable for functional genomics. Here we show a widely applicable approach to construction and use of custom bidirectional promoters capable of reproducible coexpression of two proteins. The use of a bidirectional promoter system overcomes many of the limitations of current coexpression systems such as unpredictable upstream and downstream expression ratios mediated by an internal ribosome entry site. We present examples of tetracycline-inducible, bidirectional promoter systems that produce simultaneous and rapid coinduction of two separate reporters to predictable levels and ratios. Steric blocking of transcription initiation by simple tetracycline repressors, rather than the use of transcriptional activator/repressor domain fusions, makes the system described here superior for investigating downstream transcriptional consequences of protein expression.

Encoded errors: mutations and rearrangements mediated by misalignment at repetitive DNA sequences

Mutations and rearrangements that occur by misalignment during DNA replication are frequent sources of genetic variation in bacteria. Dislocations between a replicating strand and its template at repetitive DNA sequences underlie the mechanism of these genetic events. Such misalignments can be transient or stable and can involve intramolecular or intermolecular DNA mispairing, even pairing across a replication fork. Paradoxically, these replication 'slippage' events both create and destroy repetitive sequences in bacterial genomes. This review catalogues several types of slippage errors, presents the cellular processes that act to limit them and discusses the consequences of this class of genetic events on the evolution of bacterial genomes and physiology.

The chromosome of Shigella flexneri bacteriophage Sf6: complete nucleotide sequence, genetic mosaicism, and DNA packaging

Shigella flexneri temperate bacteriophage Sf6 is of interest in part because its prophage expresses the oac gene that alters the antigenic properties of the surface O-antigen polysaccharide of its host bacterium. We have determined the complete sequence of its 39,044 bp genome. The sequence shows that Sf6 is a member of the canonical lambdoid phage group, and like other phages of this type has a highly mosaic genome. It has chromosomal regions that encode proteins >80% identical with at least 15 different previously characterized lambdoid phages and prophages, but 43% of the genome, including the virion assembly genes, is homologous to the genome of one phage, HK620. An analysis of the nucleotide differences between Sf6 and HK620 indicates that even these similar regions are highly mosaic. This mosaicism suggests ways in which the virion structural proteins might interact with each other. The Sf6 early operons are arranged like a typical lambdoid phage, with "boundary sequences" often found between functional modules in the "metabolic" genome domain. By virtue of high degree of similarity in the encoding genes and their DNA target sites, we predict that the integrase, early transcription anti-terminator, CI and Cro repressors, and CII protein of Sf6 have DNA binding specificities very similar to the homologous proteins encoded by phages HK620, lambda, 434 and P22, respectively. The late operon contains two tRNA genes. The Sf6 terminase genes are unusual. Analysis of in vivo initiation of the DNA packaging series showed that the Sf6 apparatus that recognizes DNA for packaging appears to cleave DNA for initiation of packaging series at many sites within a large region of about 1800 bp that includes a possible pac site. This is unlike previously characterized phage packaging mechanisms.

Interplay between DNA replication, recombination and repair based on the structure of RecG helicase

Recent studies in Escherichia coli indicate that the interconversion of DNA replication fork and Holliday junction structures underpins chromosome duplication and helps secure faithful transmission of the genome from one generation to the next. It facilitates interplay between DNA replication, recombination and repair, and provides means to rescue replication forks stalled by lesions in or on the template DNA. Insight into how this interconversion may be catalysed has emerged from genetic, biochemical and structural studies of RecG protein, a member of superfamily 2 of DNA and RNA helicases. We describe how a single molecule of RecG might target a branched DNA structure and translocate a single duplex arm to drive branch migration of a Holliday junction, interconvert replication fork and Holliday junction structures and displace the invading strand from a D loop formed during recombination at a DNA end. We present genetic evidence suggesting how the latter activity may provide an efficient pathway for the repair of DNA double-strand breaks that avoids crossing over, thus facilitating chromosome segregation at cell division.

Folding of a stable DNA motif involves a highly cooperative network of interactions

Hairpins are structural elements that play important roles in the folding and function of RNA and DNA. The extent of cooperativity in folding is an important aspect of the RNA folding problem. We reasoned that an investigation into the origin of cooperativity might be best carried out on a stable nucleic acid system with a limited number of interactions, such as a stable DNA hairpin loop. The stable d(cGNAg) hairpin loop motif (closing base pair in lower case; loop in upper case; N = A, C, G, or T) is stabilized through only three interactions: two loop-loop hydrogen bonds in a sheared GA base pair and a loop-closing base pair interaction. Herein, we investigate this network of interactions and test whether the loop-loop and loop-closing base pair interactions communicate. Thermodynamic measurements of nucleotide analogue substituted oligonucleotides were used to probe the additivity of the interactions. On the basis of double mutant cycles, all interactions were found to be nonadditive and interdependent, suggesting that loop-loop and loop-closing base pair interactions form in a highly cooperative manner. When double mutant cycles were repeated in the absence of the other interaction, nonadditivity was significantly reduced suggesting that coupling is indirect and requires all three interactions in order to be optimal. A cooperative network of interactions helps explain the structural and energetic bases of stability in certain DNA hairpins and paves the way for similar studies in more complex nucleic acid systems.

Genetic engineering using homologous recombination

In the past few years, in vivo technologies have emerged that, due to their efficiency and simplicity, may one day replace standard genetic engineering techniques. Constructs can be made on plasmids or directly on the Escherichia coli chromosome from PCR products or synthetic oligonucleotides by homologous recombination. This is possible because bacteriophage-encoded recombination functions efficiently recombine sequences with homologies as short as 35 to 50 base pairs. This technology, termed recombineering, is providing new ways to modify genes and segments of the chromosome. This review describes not only recombineering and its applications, but also summarizes homologous recombination in E. coli and early uses of homologous recombination to modify the bacterial chromosome. Finally, based on the premise that phage-mediated recombination functions act at replication forks, specific molecular models are proposed.

Long CTG.CAG repeat sequences markedly stimulate intramolecular recombination

Previous studies have shown that homologous recombination is a powerful mechanism for generation of massive instabilities of the myotonic dystrophy CTG.CAG sequences. However, the frequency of recombination between the CTG.CAG tracts has not been studied. Here we performed a systematic study on the frequency of recombination between these sequences using a genetic assay based on an intramolecular plasmid system in Escherichia coli. The rate of intramolecular recombination between long CTG.CAG tracts oriented as direct repeats was extraordinarily high; recombinants were found with a frequency exceeding 12%. Recombination occurred in both RecA(+) and RecA(-) cells but was approximately 2-11 times higher in the recombination proficient strain. Long CTG.CAG tracts recombined approximately 10 times more efficiently than non-repeating control sequences of similar length. The recombination frequency was 60-fold higher for a pair of (CTG.CAG)(165) tracts compared with a pair of (CTG.CAG)(17) sequences. The CTG.CAG sequences in orientation II (CTG repeats present on a lagging strand template) recombine approximately 2-4 times more efficiently than tracts of identical length in the opposite orientation relative to the origin of replication. This orientation effect implies the involvement of DNA replication in the intramolecular recombination between CTG.CAG sequences. Thus, long CTG.CAG tracts are hot spots for genetic recombination.

A 160-bp palindrome is a Rad50.Rad32-dependent mitotic recombination hotspot in Schizosaccharomyces pombe

Palindromic sequences can form hairpin and cruciform structures that pose a threat to genome integrity. We found that a 160-bp palindrome (an inverted repeat of 80 bp) conferred a mitotic recombination hotspot relative to a control nonpalindromic sequence when inserted into the ade6 gene of Schizosaccharomyces pombe. The hotspot activity of the palindrome, but not the basal level of recombination, was abolished by a rad50 deletion, by a rad50S "separation of function" mutation, or by a rad32-D25A mutation in the nuclease domain of the Rad32 protein, an Mre11 homolog. We propose that upon extrusion of the palindrome the Rad50.Rad32 nuclease complex recognizes and cleaves the secondary structure thus formed and generates a recombinogenic break in the DNA.

Instability of repetitive DNA sequences: the role of replication in multiple mechanisms

Rearrangements between tandem sequence homologies of various lengths are a major source of genomic change and can be deleterious to the organism. These rearrangements can result in either deletion or duplication of genetic material flanked by direct sequence repeats. Molecular genetic analysis of repetitive sequence instability in Escherichia coli has provided several clues to the underlying mechanisms of these rearrangements. We present evidence for three mechanisms of RecA-independent sequence rearrangements: simple replication slippage, sister-chromosome exchange-associated slippage, and single-strand annealing. We discuss the constraints of these mechanisms and contrast their properties with RecA-dependent homologous recombination. Replication plays a critical role in the two slipped misalignment mechanisms, and difficulties in replication appear to trigger rearrangements via all these mechanisms.