6 research outputs found
The Complete Genome Sequence of āCandidatus Liberibacter solanacearumā, the Bacterium Associated with Potato Zebra Chip Disease
Zebra Chip (ZC) is an emerging plant disease that causes aboveground decline of
potato shoots and generally results in unusable tubers. This disease has led to
multi-million dollar losses for growers in the central and western United States
over the past decade and impacts the livelihood of potato farmers in Mexico and
New Zealand. ZC is associated with āCandidatus
Liberibacter solanacearumā, a fastidious alpha-proteobacterium that is
transmitted by a phloem-feeding psyllid vector, Bactericera
cockerelli Sulc. Research on this disease has been hampered by a
lack of robust culture methods and paucity of genome sequence information for
āCa. L. solanacearumā. Here we present the
sequence of the 1.26 Mbp metagenome of āCa. L.
solanacearumā, based on DNA isolated from potato psyllids. The coding
inventory of the āCa. L. solanacearumā genome was
analyzed and compared to related Rhizobiaceae to better
understand āCa. L. solanacearumā physiology and
identify potential targets to develop improved treatment strategies. This
analysis revealed a number of unique transporters and pathways, all potentially
contributing to ZC pathogenesis. Some of these factors may have been acquired
through horizontal gene transfer. Taxonomically, āCa. L.
solanacearumā is related to āCa. L.
asiaticusā, a suspected causative agent of citrus huanglongbing, yet many
genome rearrangements and several gene gains/losses are evident when comparing
these two Liberibacter. species. Relative to āCa. L.
asiaticusā, āCa. L. solanacearumā probably
has reduced capacity for nucleic acid modification, increased amino acid and
vitamin biosynthesis functionalities, and gained a high-affinity iron transport
system characteristic of several pathogenic microbes
Type I restriction enzymes and their relatives
Type I restriction enzymes (REases) are large pentameric proteins with separate restriction (R), methylation (M) and DNA sequence-recognition (S) subunits. They were the first REases to be discovered and purified, but unlike the enormously useful Type II REases, they have yet to find a place in the enzymatic toolbox of molecular biologists. Type I enzymes have been difficult to characterize, but this is changing as genome analysis reveals their genes, and methylome analysis reveals their recognition sequences. Several Type I REases have been studied in detail and what has been learned about them invites greater attention. In this article, we discuss aspects of the biochemistry, biology and regulation of Type I REases, and of the mechanisms that bacteriophages and plasmids have evolved to evade them. Type I REases have a remarkable ability to change sequence specificity by domain shuffling and rearrangements. We summarize the classic experiments and observations that led to this discovery, and we discuss how this ability depends on the modular organizations of the enzymes and of their S subunits. Finally, we describe examples of Type II restrictionāmodification systems that have features in common with Type I enzymes, with emphasis on the varied Type IIG enzymes
The Orf18 Gene Product from Conjugative Transposon Tn916 Is an ArdA Antirestriction Protein that Inhibits Type I DNA Restriction-Modification Systems
Gene orf18, which is situated within the intercellular transposition region of the conjugative transposon Tn916 from the bacterial pathogen Enterococcus faecalis, encodes a putative ArdA (alleviation of restriction of DNA A) protein. Conjugative transposons are generally resistant to DNA restriction upon transfer to a new host. ArdA from Tn916 may be responsible for the apparent immunity of the transposon to DNA restriction and modification (R/M) systems and for ensuring that the transposon has a broad host range. The orf18 gene was engineered for overexpression in Escherichia coli, and the recombinant ArdA protein was purified to homogeneity. The protein appears to exist as a dimer at nanomolar concentrations but can form larger assemblies at micromolar concentrations. R/M assays revealed that ArdA can efficiently inhibit R/M by all four major classes of Type I R/M enzymes both in vivo and in vitro. These R/M systems are present in over 50% of sequenced prokaryotic genomes. Our results suggest that ArdA can overcome the restriction barrier following conjugation and so helps increase the spread of antibiotic resistance genes by horizontal gene transfer.