Evolutionary lines among Salmonella enteritidis phage types are identified by insertion sequence IS200 distribution

A survey was made of the presence, copy number and location of the Salmonella-specific DNA insertion element IS200, within the genomes of the 27 phage type strains of Salmonella enteritidis. All the phage type strains contained copies of IS200 revealed by genomic Southern blot hybridizations with a 300-bp DNA probe internal to the element. Restriction site variation around IS200 insertion sites was examined. Three fundamental patterns of hybridization corresponding to chromosomal IS200 loci were found. In terms of population genetics, these 'IS200 profiles' correspond to clonal lineages of recent evolutionary origin, and underline the phage-typing scheme for epidemiological subdivision of S. enteritidis. The molecular analysis is consistent with genetic selection pressures which are apparent in the observed epidemiological distribution of S. enteritidis, since each clonal lineage contained one of the phage types of major clinical importance in the U.K.     

Unravelling the Leishmania genome

The past few years have seen significant advances in our understanding of eukaryotic genomes. In the field of parasitology, this is best exemplified by the application of genome mapping techniques to the study of genome structure and function in the protozoan parasite, Leishmania. Although much is known about the organism and the diseases it causes, molecular genetics has only recently begun to play a major part in elucidating some of the unusual characteristics of this interesting parasite. Mapping of the small (35 Mb) genome and determination of the functional role of genes by the application of in vitro homologous gene targeting techniques are revealing novel avenues for the development of prophylactic measures.     

Centrosomes and cancer.

The centrosome functions as the major microtubule organizing center (MTOC) of the cell and as such it determines the number, polarity, and organization of interphase and mitotic microtubules. Cytoplasmic organization, cell polarity and the equal partition of chromosomes into daughter cells at the time of cell division are all dependent on the normal function of the centrosome and on its orderly duplication, once and only once, in each cell cycle. Malignant tumor cells show characteristic defects in cell and tissue architecture and in chromosome number that can be attributed to inappropriate centrosome behavior during tumor progression. In this review, we will summarize recent observations linking centrosome defects to disruption of normal cell and tissue organization and to chromosomal instability found in malignant tumors.     

Yeast ARMs (DNA at-risk motifs) can reveal sources of genome instability

The genomes of all organisms contain an abundance of DNA repeats which are at-risk for causing genetic change. We have used the yeast Saccharomyces cerevisiae to investigate various repeat categories in order to understand their potential for causing genomic instability and the role of DNA metabolism factors. Several types of repeats can increase enormously the likelihood of genetic changes such as mutation or recombination when present either in wild type or mutants defective in replication or repair. Specifically, we have investigated inverted repeats, homonucleotide runs, and short distant repeats and the consequences of various DNA metabolism mutants. Because the at-risk motifs (ARMs) that we characterized are sensitive indicators, we have found that they are useful tools to reveal new genetic factors affecting genome stability as well as to distinguish subtle differences between alleles.     

Single copy DNA homology in sea stars

Summary The sequence homology in the single copy DNA of sea stars has been measured. Labeled single copy DNA fromPisaster ochraceus was reannealed with excess genomic DNA fromP. brevispinus, Evasterias troschelii, Pycnopodia helianthoides, Solaster stimpsoni, andDermasterias imbricata. Reassociation reactions were performed under two criteria of salt and temperature. The extent of reassociation and thermal denaturation characteristics of hybrid single copy DNA molecules follow classical taxonomic lines.P. brevispinus DNA contains essentially all of the sequences present inP. ochraceus single copy tracer whileEvasterias andPycnopodia DNAs contain 52% and 46% of such sequences respectively. Reciprocal reassociation reactions with labeledEvasterias single copy DNA confirm the amount and fidelity of the sequence homology. There is a small definite reaction of uncertain homology betweenP. ochraceus single copy DNA andSolaster orDermasterias DNA. SimilarlySolaster DNA contains sequences homologous to approximately 18% ofDermasterias unique DNA. The thermal denaturation temperatures of heteroduplexes indicate that the generaPisaster andEvasterias diverged shortly after the divergence of the subfamilies Pycnopodiinae and Asteriinae. The twoPisaster species diverged more recently, probably in the most recent quarter of the interval since the separation of the generaPisaster andEvasterias.     

Genome analysis of Pseudomonas aeruginosa by field inversion gel electrophoresis

Abstract The genome of Pseudomonas aeruginosa was analysed by digestion with rare-cutting restriction endonucleases and subsequent field inversion gel electrophoresis (FIGE). P. aeruginosa strain PAO and the 17 IATS strains were investigated. Each strain displayed a unique pattern of restriction fragments. Digestion with Dra I and Ssp I yielded, respectively 7–11 and 2–5 fragments of more than 130 kb in size, indicating the non-random occurrence of AT-rich sequences in the P. aeruginosa genome. The genome size of P. aeruginosa PAO was estimated to be (2.2 ± 0.3) × 10⁶ bp. The applications of DNA fingerprinting for gene cloning, construction of a physical chromosome map, and epidemiological studies, are discussed.     

Genetic instability during embryogenic cloning of celery

Genetically marked tissues of celery (Apium graveolens) were employed to contrast genetic and chromosomal stability in serially bulk-transferred callus and regenerated plants. After six months in culture, 84% of the callus cells were karologically indistinguishable from normal, while the remainder exhibited chromosome loss and/or fusion. All of 50 clones derived from this tissue expressed the control phenotype with respect to heterozygous isozyme markers. Of 95 plants regenerated from the same tissue, 94 were phenotypically indistinguishable from the original explant donor, and cytogenetic analyses revealed the presence in 4.3% of an accessory chromosome, while the remainder were normal diploids. Analysis of the selfed progeny of these regenerated plants revealed the presence of a new recessive mutation causing abnormal leaf morphology at a frequency of 1.8%. Only one of 40 cells in 12-month-old callus tissue was karyologically indistinguishable from normal, the remainder consisting primarily of hypodiploids. The observation that all 50 clones were phenotypically heterozygous was statistically inconsistent with the hypothesis that hypodiploidy was associated with random complete chromosome loss. The culture had, at this point, lost the ability to regenerate. It is speculated that embryogenic cloning of celery may be suitable under certain circumstances for direct field establishment, but that levels of new genetic variation are sufficiently high to preclude its use for seed production.     

Combined loss of three DNA damage response pathways renders C. elegans intolerant to light

Infliction of DNA damage initiates a complex cellular reaction – the DNA damage response – that involves both signaling and DNA repair networks with many redundancies and parallel pathways. Here, we reveal the three strategies that the simple multicellular eukaryote, C. elegans, uses to deal with DNA damage induced by light. Separately inactivating repair or replicative bypass of photo-lesions results in cellular hypersensitivity towards UV-light, but impeding repair of replication associated DNA breaks does not. Yet, we observe an unprecedented synergistic relationship when these pathways are inactivated in combination. C. elegans mutants that lack nucleotide excision repair (NER), translesion synthesis (TLS) and alternative end joining (altEJ) grow undisturbed in the dark, but become sterile when grown in light. Even exposure to very low levels of normal daylight impedes animal growth. We show that NER and TLS operate to suppress the formation of lethal DNA breaks that require polymerase theta-mediated end joining (TMEJ) for their repair. Our data testifies to the enormous genotoxicity of light and to the demand of multiple layers of protection against an environmental threat that is so common.