Biomedical subjects

D S Guttman

Publications and source records attributed to D S Guttman.

8 recordsLinked to original sources

PubMed2001

Functional analysis of the type III effectors AvrRpt2 and AvrRpm1 of Pseudomonas syringae with the use of a single-copy genomic integration system.

Gram-negative phytopathogenic bacteria require a type III secretion apparatus for pathogenesis, presumably to deliver Avr effector proteins directly into plant cells. To extend previous studies of Avr effectors that employed plasmids encoding Avr proteins, we developed a system that permits the integration of any gene into the Pseudomonas syringae genome in single copy. With this system, we confirmed earlier findings showing that P. syringae pv. maculicola strain PsmES4326 expressing the AvrRpt2 effector induces a resistance response in plants with the cognate R gene, RPS2. Chromosomally located avrRpt2, however, provoked a stronger resistance response than that observed with plasmid-expressed AvrRpt2 in RPS2+ plants. Additionally, chromosomal expression of AvrRpt2 conferred a fitness advantage on P. syringae grown in rps2- plants, aiding in growth within leaves and escape to leaf surfaces that was difficult to detect with plasmid-borne avrRpt2. Finally, with the use of the genomic integration system, we found that a chimeric protein composed of the N terminus of the heterologous AvrRpml effector and the C-terminal effector region of AvrRpt2 was delivered to plant cells. Because the C terminus of AvrRpt2 cannot translocate into plant cells on its own, this indicates that the N-terminal region can direct secretion and translocation during an infection, which supports the view that Avr proteins have a modular design. This work establishes a readily manipulatable system to study type III effectors in a biologically realistic context.

Arabidopsis↗

PubMed1999

The gain-of-function Arabidopsis acd6 mutant reveals novel regulation and function of the salicylic acid signaling pathway in controlling cell death, defenses, and cell growth.

We isolated a dominant gain-of-function Arabidopsis mutant, accelerated cell death 6 (acd6), with elevated defenses, patches of dead and enlarged cells, reduced stature, and increased resistance to Pseudomonas syringae. The acd6-conferred phenotypes are suppressed by removing a key signaling molecule, salicylic acid (SA), by using the nahG transgene, which encodes SA hydroxylase. This suppression includes phenotypes that are not induced by application of SA to wild-type plants, indicating that SA acts with a second signal to cause many acd6-conferred phenotypes. acd6-nahG plants show hyperactivation of all acd6-conferred phenotypes after treatment with a synthetic inducer of the SA pathway, benzo(1,2, 3)thiadiazole-7-carbothioic acid (BTH), suggesting that SA acts with and also modulates the levels and/or activity of the second defense signal. acd6 acts partially through a NONEXPRESSOR OF PR 1 (NPR1) gene-independent pathway that activates defenses and confers resistance to P. syringae. Surprisingly, BTH-treated acd6-nahG plants develop many tumor-like abnormal growths, indicating a possible role for SA in modulating cell growth.

Arabidopsis↗

PubMed1998

An X-linked gene with a degenerate Y-linked homologue in a dioecious plant.

Most flowering plants are hermaphroditic, having flowers with both male and female parts. Less than 4% of plant species are dioecious (with individuals of separate sexes), and many of these species have chromosome-mediated sex determination. The taxonomic distribution of separate sexes and chromosomal sex-determination systems in the flowering plants indicates that plant sex chromosomes have evolved recently through replicated, independent events, contrasting with the ancient origins of mammalian and insect sex chromosomes. Plant sex chromosomes, therefore, offer opportunities to study the most interesting early stages of the evolution of sex chromosomes. Here we show that a gene encoding a male-specific protein is linked to the X chromosome in the dioecious plant Silene latifolia, and that it has a degenerate homologue in the non-pairing region of the Y chromosome. The Y-linked locus has degenerated as a result of nucleotide deletion and the accumulation of repetitive sequences. We have identified both the first X-linked gene and the first pair of homologous sex-linked loci to be found in plants. The homology between the active X-linked locus and the degenerate Y-linked locus supports a common ancestry for these two loci.

Base Sequence↗

PubMed1997

Seeing selection in S allele sequences.

New data on allelic sequence diversity in natural populations provide evidence for natural selection acting on the self-incompatibility loci of two plant species; there are interesting parallels with, and differences from, other polymorphic systems such as mammalian MHC loci.

Alleles↗

PubMed1997

Lack of degeneration of loci on the neo-Y chromosome of Drosophila americana americana.

The extent of genetic degeneration of the neo-Y chromosome of Drosophila americana americana has been investigated. Three loci, coding for the enzymes enolase, phosphoglycerate kinase and alcohol dehydrogenase, have been localized to chromosome 4 of D. a. americana, which forms the neo-Y and neo-X chromosomes. Crosses between D.a. americana and D. virilis or D. montana showed that the loci coding for these enzymes carry active alleles on the neo-Y chromosome in all wild-derived strains of americana that were tested. Intercrosses between a genetically marked stock of virilis and strains of americana were carried out, creating F3 males that were homozygous for sections of the neo-Y chromosome. The sex ratios in the F3 generation of the intercrosses showed that no lethal alleles have accumulated on any of the neo-Y chromosomes tested. There was evidence for more minor reductions in fitness, but this seems to be mainly caused by deleterious alleles that are specific to each strain. A similar picture was provided by examination of the segregation ratios of two marker genes among the F3 progeny. Overall, the data suggest that the neo-Y chromosome has undergone very little degeneration, certainly not to the extent of having lost the functions of vital genes. This is consistent with the recent origin of the neo-Y and neo-X chromosomes, and the slow rates at which the forces that cause Y chromosome degeneration are likely to work.

Animals↗

PubMed1996

Multiple infections of Ixodes scapularis ticks by Borrelia burgdorferi as revealed by single-strand conformation polymorphism analysis.

The genetic heterogeneity of the spirochete Borrelia burgdorferi within single adult black-legged ticks from Shelter Island, N.Y., was determined by cold, single-strand conformation polymorphism (SSCP) analysis. The central region of the ospA gene of B. burgdorferi from infected ticks was amplified by nested PCR. Amplified product of the correct size was obtained from 20 to 45 ticks (44%). This is the fraction of ticks that is expected to be infected with B. burgdorferi. Four variant classes were determined by SSCP analysis. Eight ticks were infected with a single variant, nine ticks were infected with two variants, two ticks were infected with three variants, and one tick was infected with all four variants. DNA from each variant was sequenced. Five different sequences were found. The sequence of each variant was different from that of another variant by a single base. SSCP analysis could distinguish three of the four single-base changes found in the region.

Animals↗

PubMed1994

Clonal divergence in Escherichia coli as a result of recombination, not mutation.

Nucleotide sequence analysis was performed on 12 natural isolates of Escherichia coli in four loci located in close proximity on the chromosome. A comparison of gene genealogies indicated that three recombination events have occurred in a subset of the strains (ECOR group A) in the time since their divergence from a common ancestor, while during the same time, no mutational divergence has occurred. The common ancestor of this subset existed no more than 2400 years ago, and recombination was shown to occur at a rate of 5.0 x 10(-9) changes per nucleotide per generation--50-fold higher than the mutation rate. Thus, recombination has been the dominant force driving the clonal divergence of the ECOR group A strains and must be considered a significant factor in structuring E. coli populations.

Biological Evolution↗

PubMed1994

Detecting selective sweeps in naturally occurring Escherichia coli.

The nucleotide sequences of the gapA and pabB genes (separated by approximately 32.5 kb) were determined in 12 natural isolates of Escherichia coli. Three analyses were performed on the data. First, the levels of polymorphism at the loci were compared within and between E. coli and Salmonella strains relative to their degrees of constraint. Second, the gapA and pabB loci were analyzed by the Hudson-Kreitman-Aguadé (HKA) test for selective neutrality. Four additional dispersed genes (crr, putP, trp and gnd) were added to the analysis to provide the necessary frame of reference. Finally, the gene genealogies of gapA and pabB were examined for topological consistency within and between the loci. These lines of evidence indicate that some evolutionary event has recently purged the variability in the region surrounding the gapA and pabB loci in E. coli. This can best be explained by the spread of a selected allele through the global E. coli population by directional selection and the resulting loss in variability in the surrounding regions due to genetic hitchhiking.

Alleles↗