Hongshing Lai

Genome sequence of Streptococcus mutans UA159, a cariogenic dental pathogen

Streptococcus mutans is the leading cause of dental caries (tooth decay) worldwide and is considered to be the most cariogenic of all of the oral streptococci. The genome of S. mutans UA159, a serotype c strain, has been completely sequenced and is composed of 2,030,936 base pairs. It contains 1,963 ORFs, 63% of which have been assigned putative functions. The genome analysis provides further insight into how S. mutans has adapted to surviving the oral environment through resource acquisition, defense against host factors, and use of gene products that maintain its niche against microbial competitors. S. mutans metabolizes a wide variety of carbohydrates via nonoxidative pathways, and all of these pathways have been identified, along with the associated transport systems whose genes account for almost 15% of the genome. Virulence genes associated with extracellular adherent glucan production, adhesins, acid tolerance, proteases, and putative hemolysins have been identified. Strain UA159 is naturally competent and contains all of the genes essential for competence and quorum sensing. Mobile genetic elements in the form of IS elements and transposons are prominent in the genome and include a previously uncharacterized conjugative transposon and a composite transposon containing genes for the synthesis of antibiotics of the gramicidin/bacitracin family; however, no bacteriophage genomes are present.

Introns and splicing elements of five diverse fungi.

ABSTRACT Genomic sequences and expressed sequence tag data for a diverse group of fungi (Saccharomyces cerevisiae, Schizosaccharomyces pombe, Aspergillus nidulans, Neurospora crassa, and Cryptococcus neoformans) provided the opportunity to accurately characterize conserved intronic elements. An examination of large intron data sets revealed that fungal introns in general are short, that 98% or more of them belong to the canonical splice site (ss) class (5′GU…AG3′), and that they have polypyrimidine tracts predominantly in the region between the 5′ ss and the branch point. Information content is high in the 5′ ss, branch site, and 3′ ss regions of the introns but low in the exon regions adjacent to the introns in the fungi examined. The two yeasts have broader intron length ranges and correspondingly higher intron information content than the other fungi. Generally, as intron length increases in the fungi, so does intron information content. Homologs of U2AF spliceosomal proteins were found in all species except for S. cerevisiae, suggesting a nonconventional role for U2AF in the absence of canonical polypyrimidine tracts in the majority of introns. Our observations imply that splicing in fungi may be different from that in vertebrates and may require additional proteins that interact with polypyrimidine tracts upstream of the branch point. Theoretical protein homologs for Nam8p and TIA-1, two proteins that require U-rich regions upstream of the branch point to function, were found. There appear to be sufficient differences between S. cerevisiae and S. pombe introns and the introns of two filamentous members of the Ascomycota and one member of the Basidiomycota to warrant the development of new model organisms for studying the splicing mechanisms of fungi.

Ecogenomics: using massively parallel pyrosequencing to understand virus ecology

Environmental samples have been analysed for viruses in metagenomic studies, but these studies have not linked individual viruses to their hosts. We designed a strategy to isolate double‐stranded RNA, a hallmark of RNA virus infection, from individual plants and convert this to cDNA with a unique four nucleotide Tag at each end. Using 96 different Tags allowed us to pool samples and still retain the link to the original sample. We then analysed the sequence of pooled samples using massively parallel sequencing with Roche 454 pyrosequencing such that 384 samples could be assessed per picotiter plate. Using this method we have been able to analyse thousands of plants, and we have discovered several thousand new plant viruses, all linked to their specific plant hosts. Here we describe the method in detail, including the results and analysis for eight pools of samples. This technology will be extremely useful in understanding the full scope of plant virus biodiversity.

Differential Accumulation of Retroelements and Diversification of NB-LRR Disease Resistance Genes in Duplicated Regions following Polyploidy in the Ancestor of Soybean

The genomes of most, if not all, flowering plants have undergone whole genome duplication events during their evolution. The impact of such polyploidy events is poorly understood, as is the fate of most duplicated genes. We sequenced an approximately 1 million-bp region in soybean (Glycine max) centered on the Rpg1-b disease resistance gene and compared this region with a region duplicated 10 to 14 million years ago. These two regions were also compared with homologous regions in several related legume species (a second soybean genotype, Glycine tomentella, Phaseolus vulgaris, and Medicago truncatula), which enabled us to determine how each of the duplicated regions (homoeologues) in soybean has changed following polyploidy. The biggest change was in retroelement content, with homoeologue 2 having expanded to 3-fold the size of homoeologue 1. Despite this accumulation of retroelements, over 77% of the duplicated low-copy genes have been retained in the same order and appear to be functional. This finding contrasts with recent analyses of the maize (Zea mays) genome, in which only about one-third of duplicated genes appear to have been retained over a similar time period. Fluorescent in situ hybridization revealed that the homoeologue 2 region is located very near a centromere. Thus, pericentromeric localization, per se, does not result in a high rate of gene inactivation, despite greatly accelerated retrotransposon accumulation. In contrast to low-copy genes, nucleotide-binding-leucine-rich repeat disease resistance gene clusters have undergone dramatic species/homoeologue-specific duplications and losses, with some evidence for partitioning of subfamilies between homoeologues.

Replication of Nonautonomous Retroelements in Soybean Appears to Be Both Recent and Common

Retrotransposons and their remnants often constitute more than 50% of higher plant genomes. Although extensively studied in monocot crops such as maize (Zea mays) and rice (Oryza sativa), the impact of retrotransposons on dicot crop genomes is not well documented. Here, we present an analysis of retrotransposons in soybean (Glycine max). Analysis of approximately 3.7 megabases (Mb) of genomic sequence, including 0.87 Mb of pericentromeric sequence, uncovered 45 intact long terminal repeat (LTR)-retrotransposons. The ratio of intact elements to solo LTRs was 8:1, one of the highest reported to date in plants, suggesting that removal of retrotransposons by homologous recombination between LTRs is occurring more slowly in soybean than in previously characterized plant species. Analysis of paired LTR sequences uncovered a low frequency of deletions relative to base substitutions, indicating that removal of retrotransposon sequences by illegitimate recombination is also operating more slowly. Significantly, we identified three subfamilies of nonautonomous elements that have replicated in the recent past, suggesting that retrotransposition can be catalyzed in trans by autonomous elements elsewhere in the genome. Analysis of 1.6 Mb of sequence from Glycine tomentella, a wild perennial relative of soybean, uncovered 23 intact retroelements, two of which had accumulated no mutations in their LTRs, indicating very recent insertion. A similar pattern was found in 0.94 Mb of sequence from Phaseolus vulgaris (common bean). Thus, autonomous and nonautonomous retrotransposons appear to be both abundant and active in Glycine and Phaseolus. The impact of nonautonomous retrotransposon replication on genome size appears to be much greater than previously appreciated.

Genotype and Phenotypes of an Intestine-Adapted Escherichia coli K-12 Mutant Selected by Animal Passage for Superior Colonization

ABSTRACT We previously isolated a spontaneous mutant of Escherichia coli K-12, strain MG1655, following passage through the streptomycin-treated mouse intestine, that has colonization traits superior to the wild-type parent strain (M. P. Leatham et al., Infect. Immun. 73:8039–8049, 2005). This intestine-adapted strain (E. coli MG1655*) grew faster on several different carbon sources than the wild type and was nonmotile due to deletion of the flhD gene. We now report the results of several high-throughput genomic analysis approaches to further characterize E. coli MG1655*. Whole-genome pyrosequencing did not reveal any changes on its genome, aside from the deletion at the flhDC locus, that could explain the colonization advantage of E. coli MG1655*. Microarray analysis revealed modest yet significant induction of catabolic gene systems across the genome in both E. coli MG1655* and an isogenic flhD mutant constructed in the laboratory. Catabolome analysis with Biolog GN2 microplates revealed an enhanced ability of both E. coli MG1655* and the isogenic flhD mutant to oxidize a variety of carbon sources. The results show that intestine-adapted E. coli MG1655* is more fit than the wild type for intestinal colonization, because loss of FlhD results in elevated expression of genes involved in carbon and energy metabolism, resulting in more efficient carbon source utilization and a higher intestinal population. Hence, mutations that enhance metabolic efficiency confer a colonization advantage.

EST library sequencing of genes expressed during early limb regeneration in the fiddler crab and transcriptional responses to ecdysteroid exposure in limb bud explants.

We have constructed directional and randomly primed cDNA libraries from mRNAs isolated during progressive stages of fiddler crab (Uca pugilator) limb regeneration. Data from these libraries are being assembled into an on-line database (http://www.genome.ou.edu/crab.html) that is both BLAST and keyword searchable; the data set is also available through GenBank. The first characterized library was made from mRNA isolated 4 days post-autotomy, when the first sign of morphological differentiation, cuticle secretion, is observed. Analysis of 1698 cDNA clones led to assignment of 473 contigs and 417 singlets, for a total of 890 sequences. Of these, ∼86% showed no assignments to characterized genes on database searching, while 14% could be assigned to a known ortholog in the COG (Clusters of Orthologous Groups) database. BLAST searches to specific protein domains in the Gene Ontology database led to assignments for ∼40% of the assembled sequences. Sequence similarity searches of other crustacean EST databases produced hits to 13-30% of the Uca query sequences. The ESTs include several genes that may be potentially ecdysteroid-responsive, such as homologs to chaperone proteins and cuticle protein genes, as well as homologs to arthropod proteins involved in retinoid/terpenoid metabolism. We have tested 3 potential candidate genes for their ability to be induced by ecdysteroid in limb bud explants; an arthropodial cuticle protein gene, and the nuclear receptor genes EcR and RXR. A subset of early blastemal limb buds (8 days post autotomy) show a positive response to ecdysteroid by 1-1.5 h, followed by a decrease in transcript abundance at longer periods of sustained incubation. Later stage buds (12 days post autotomy-late premolt) show decreases in steady-state mRNA levels by 1.5 h, or are completely refractory to ecdysteroid exposure.

A plea to search for deletion polymorphism through genome scans in populations.

We thank M. Wigler and N. Lisitsyn for having made available the R271 primers, U.J. Kim for screening the BAC-contig panel of chromosome 22 and for providing BAC 322F3 and A. Baldini, D. Beck, B. Emauel and L. Luzzatto for their constructive criticism. This work was funded in part by grants from the Italian Ministry of Research, MURST, Italy (to M.S. and L.C.) and from the NIH/NHGRI and NSF EPSCOR program (to B.A.R).