RecA-independent high-frequency deletion of recombinant cosmid DNA in Escherichia coli

Process of reductive evolution during 10 years in plasmids of a non-insect-transmissible phytoplasma

A non-insect-transmissible phytoplasma strain (OY-NIM) was obtained from insect-transmissible strain OY-M by plant grafting using no insect vectors. In this study, we analyzed for the gene structure of plasmids during its maintenance in plant tissue culture for 10 years. OY-M strain has one plasmid encoding orf3 gene which is thought to be involved in insect transmissibility. The gradual loss of OY-NIM plasmid sequence was observed in subsequent steps: first, the promoter region of orf3 was lost, followed by the loss of then a large region including orf3, and finally the entire plasmid was disappeared. In contrast, no mutation was found in a pseudogene on OY-NIM chromosome in the same period, indicating that OY-NIM plasmid evolved more rapidly than the chromosome-encoded gene tested. Results revealed an actual evolutionary process of OY plasmid, and provide a model for the stepwise process in reductive evolution of plasmids by environmental adaptation. Furthermore, this study indicates the great plasticity of plasmids throughout the evolution of phytoplasma.

Bacterial genome size reduction by experimental evolution

Bacterial evolution toward endosymbiosis with eukaryotic cells is associated with extensive bacterial genome reduction and loss of metabolic and regulatory capabilities. Here we examined the rate and process of genome reduction in the bacterium Salmonella enterica by a serial passage experimental evolution procedure. The initial rate of DNA loss was estimated to be 0.05 bp per chromosome per generation for a WT bacterium and approximately 50-fold higher for a mutS mutant defective in methyl-directed DNA mismatch repair. The endpoints were identified for seven chromosomal deletions isolated during serial passage and in two separate genetic selections. Deletions ranged in size from 1 to 202 kb, and most of them were not associated with DNA repeats, indicating that they were formed via RecA-independent recombination events. These results suggest that extensive genome reduction can occur on a short evolutionary time scale and that RecA-dependent homologous recombination only plays a limited role in this process of jettisoning superfluous DNA.

Assembly of large genomic segments in artificial chromosomes by homologous recombination in Escherichia coli

We developed a method for the reconstruction of a 100 kb DNA fragment into a bacterial artificial chromosome (BAC). The procedure makes use of iterative rounds of homologous recombination in Escherichia coli. Smaller, overlapping fragments of cloned DNA, such as cosmid clones, are required. They are transferred first into a temperature-sensitive replicon and then into the BAC of choice. We demonstrated the usefulness of this procedure by assembling a 90 kb genomic segment into an E.coli-STREPTOMYCES: artificial chromosome (ESAC). Using this procedure, ESACs are easy to handle and remarkably more stable than the starting cosmids.

Iterative selection from a Salmonella typhimurium cosmid library can lead to the isolation of an atypically small plasmid

We have constructed a DNA library from a virulent Salmonella typhimurium strain, in an avirulent strain. The process of selecting the components of interest from the library involved iterative growth in liquid culture. This resulted, after four cycles, in the culture becoming homogeneous for a single plasmid, which was much smaller than the average size for the library. We have identified the larger precursor of this plasmid which has two regions of sufficient homology to allow recombination resulting in the formation of the small plasmid. S. typhimurium carrying the small plasmid have a smaller genetic burden than other members of the library and survive better in spent culture medium, facilitating selection on repeated subculture. Such rapid adventitious selection has important implications for isolation of clones of interest from genomic libraries.

LTR-directed homologous recombination of full-length HIV-1 provirus clone in recA(-) bacteria

During molecular cloning of full-length retroviral plasmid clones occurrence of homologous recombination (HR) between LTR regions is frequently observed. In order to evaluate appropriate host bacterial strains for cloning such HR-prone plasmids, we utilized a linearized template plasmid containing a full-length HIV-1 proviral sequence. The plasmid was linearized within the viral sequence so that plasmid transformed bacteria would grow only when the plasmid was circularized by HR. Using this genetic system for detecting HR, we evaluated the frequency of HR in various recA(-) bacterial strains which are commercially available and in some recA-null strains in which recA-defective phenotype was constructed by P1 transduction. We found that HR occurred even in recA-null strains although in lesser frequencies. The nucleotide sequence analysis at the junction of recombination revealed no loss, insertion or duplication of DNA sequence. It is suggested that recombination machinery other than the RecA system is involved.

Sequence specific generation of a DNA panhandle permits PCR amplification of unknown flanking DNA

We present a novel method for the PCR amplification of unknown DNA that flanks a known segment directly from human genomic DNA. PCR requires that primer annealing sites be present on each end of the DNA segment that is to be amplified. In this method, known DNA is placed on the uncharacterized side of the sequence of interest via DNA polymerase mediated generation of a PCR template that is shaped like a pan with a handle. Generation of this template permits specific amplification of the unknown sequence. Taq (DNA) polymerase was used to form the original template and to generate the PCR product. 2.2 kb of the beta-globin gene, and 657 bp of the 5' flanking region of the cystic fibrosis transmembrane conductance regulator gene, were amplified directly from human genomic DNA using primers that initially flank only one side of the region amplified. This method will provide a powerful tool for acquiring DNA sequence information.

Site-directed, recombination-mediated mutagenesis of a complex gene locus

We have generated a site-specific 17 bp insertion within a 38 kb chick globin gene cluster by employing the recombination abilities of Saccharomyces cerevisiae. This gene cluster contains four beta-type globin genes which share a high degree of sequence homology. In this procedure, a small fragment of beta A-globin DNA containing a 17 bp insertion is subcloned into a URA3-based yeast integrating vector (YIp). This mutated globin subclone is introduced into cells that carry the 38 kb globin cluster clone on a single-copy, circular vector derived from a yeast artificial chromosome (YAC). Insertion of the 17 bp oligomer is achieved by targeted integration of the Ylp subclone. The recombinant contains the normal beta A-globin gene, the mutant gene and Ylp vector sequences between the two copies. Excision of the vector sequences and one copy of the duplicated globin sequences by homologous recombination is required for cell survival when exposed to the selective agent 5-fluoroorotic acid, which is toxic to ura+ yeast cells. Depending upon the point of the cross-over, a ura- yeast cell bearing either a wild-type globin gene or a 17 bp insertion mutation will result. By restriction mapping and in vitro transcription analysis, the beta A-globin gene containing the 17 bp insert has no nonspecific mutations generated during the recombination and selection procedures. Specific mutations of regulatory regions, including protein-DNA binding sites, can be accurately targeted within extensive DNA clones by this method.