A hidden Markov support vector machine framework incorporating profile geometry learning for identifying microbial RNA in tiling array data

Characterization of a Signaling System in Streptococcus mitis That Mediates Interspecies Communication with Streptococcus pneumoniae

Streptococcus mitis is found in the oral cavity and nasopharynx and forms a significant portion of the human microbiome. In this study, in silico analyses indicated the presence of an Rgg regulator and short hydrophobic peptide (Rgg/SHP) cell-to-cell communication system in S. mitis Although Rgg presented greater similarity to a repressor in Streptococcus pyogenes, autoinducing assays and genetic mutation analysis revealed that in S. mitis Rgg acts as an activator. Transcriptome analysis showed that in addition to shp, the system regulates two other downstream genes, comprising a segment of a putative lantibiotic gene cluster that is in a conjugative element locus in different members of the mitis group. Close comparison to a similar lantibiotic gene cluster in Streptococcus pneumoniae indicated that S. mitis lacked the full set of genes. Despite the potential of SHP to trigger a futile cycle of autoinduction, growth was not significantly affected for the rgg mutant under normal or antibiotic stress conditions. The S. mitis SHP was, however, fully functional in promoting cross-species communication and increasing S. pneumoniae surface polysaccharide production, which in this species is regulated by Rgg/SHP. The activity of SHPs produced by both species was detected in cocultures using a S. mitis reporter strain. In competitive assays, a slight advantage was observed for the rgg mutants. We conclude that the Rgg/SHP system in S. mitis regulates the expression of its own shp and activates an Rgg/SHP system in S. pneumoniae that regulates surface polysaccharide synthesis. Fundamentally, cross-communication of such systems may have a role during multispecies interactions.IMPORTANCE Bacteria secrete signal molecules into the environment which are sensed by other cells when the density reaches a certain threshold. In this study, we describe a communication system in Streptococcus mitis, a commensal species from the oral cavity, which we also found in several species and strains of streptococci from the mitis group. Further, we show that this system can promote cross-communication with S. pneumoniae, a closely related major human pathogen. Importantly, we show that this cross-communication can take place during coculture. While the genes regulated in S. mitis are likely part of a futile cycle of activation, the target genes in S. pneumoniae are potentially involved in virulence. The understanding of such complex communication networks can provide important insights into the dynamics of bacterial communities.

Identification and analysis of the cleavage site in a signal peptide using SMOTE, dagging, and feature selection methods

Several machine learning algorithms were adopted to investigate cleavage sites in a signal peptide. An optimal dagging based classifier was constructed and 870 important features were deemed to be important for this classifier.

A Quorum-Sensing System That Regulates Streptococcus pneumoniae Biofilm Formation and Surface Polysaccharide Production

Quorum sensing regulates bacterial social behaviors by production, secretion, and sensing of pheromones. In this study, we characterized a new quorum-sensing system of the Rgg/SHP class in S. pneumoniae D39. The system was found to directly induce the expression of a single gene cluster comprising the gene for the SHP pheromone and genes with putative functions in capsule synthesis. Capsule size, as measured by dextran exclusion, was increased by SHP exposure in R36A, an unencapsulated derivative of D39. In the encapsulated parent strain, overexpression of the gene cluster increased capsule size, supporting the role of Rgg/SHP in the synthesis of surface polysaccharides. Further, we found that biofilm formation on epithelial cells was reduced by overexpression of the system and increased in a mutant with an rgg deletion. Placing surface polysaccharide expression under quorum-sensing regulation may enable S. pneumoniae to tune interactions with the host and other bacteria in accordance with environmental and cell density conditions.

Despite vaccines, Streptococcus pneumoniae kills more than a million people yearly. Thus, understanding how pneumococci transition from commensals to pathogens is particularly relevant. Quorum sensing regulates collective behaviors and thus represents a potential driver of commensal-to-pathogen transitions. Rgg/small hydrophobic peptide (SHP) quorum-sensing systems are widespread in streptococci, yet they remain largely uncharacterized in S. pneumoniae. Using directional transcriptome sequencing, we show that the S. pneumoniae D39 Rgg0939/SHP system induces the transcription of a single gene cluster including shp and capsule gene homologs. Capsule size measurements determined by fluorescein isothiocyanate-dextran exclusion allowed assignment of the system to the regulation of surface polysaccharide expression. We found that the SHP pheromone induced exopolysaccharide expression in R36A, an unencapsulated derivative of D39. In the encapsulated parent strain, overexpression of the Rgg system resulted in a mutant with increased capsule size. In line with previous studies showing that capsule expression is inversely associated with biofilm formation, we found that biofilm formed on lung epithelial cells was decreased in the overexpression strain and increased in an rgg deletion mutant. Although no significant differences were observed between D39 and the rgg deletion mutant in a mouse model of lung infection, in competitive assays, overexpression reduced fitness. This is the first study to reveal a quorum-sensing system in streptococci that regulates exopolysaccharide synthesis from a site distinct from the original capsule locus.

IMPORTANCE Quorum sensing regulates bacterial social behaviors by production, secretion, and sensing of pheromones. In this study, we characterized a new quorum-sensing system of the Rgg/SHP class in S. pneumoniae D39. The system was found to directly induce the expression of a single gene cluster comprising the gene for the SHP pheromone and genes with putative functions in capsule synthesis. Capsule size, as measured by dextran exclusion, was increased by SHP exposure in R36A, an unencapsulated derivative of D39. In the encapsulated parent strain, overexpression of the gene cluster increased capsule size, supporting the role of Rgg/SHP in the synthesis of surface polysaccharides. Further, we found that biofilm formation on epithelial cells was reduced by overexpression of the system and increased in a mutant with an rgg deletion. Placing surface polysaccharide expression under quorum-sensing regulation may enable S. pneumoniae to tune interactions with the host and other bacteria in accordance with environmental and cell density conditions.

A positive feedback loop mediated by Sigma X enhances expression of the streptococcal regulator ComR

Natural transformation is used by bacteria to take up DNA from their surroundings and incorporate it into their genomes. Streptococci do so during a transient period of competence, triggered by pheromones that they produce, secrete and sense under conditions influenced by the environment. In Streptococcus mutans, Streptococcus suis, and species of the bovis, salivarius and pyogenic groups of streptococci, the pheromone XIP is sensed by the intra-cellular regulator ComR, that in turn activates the transcription of comS, encoding the XIP precursor, and of sigX, encoding the only known alternative sigma factor in streptococci. Although induction of comR during competence has been known for more than fifteen years, the mechanism regulating its expression remains unidentified. By a combination of directional RNA-sequencing, optimal competence conditions, stepwise deletions and marker-less genome editing, we found that SigX is the missing link in overproduction of ComR. In the absence of comR induction, both sigX expression and transformation were significantly reduced. Placing comR and comS transcripts under the control of different regulators so as to form two interlocked positive feedback circuits may enable S. mutans to fine-tune the kinetics and magnitude of the competence response according to their need.

Comprehensive Transcriptome Profiles of Streptococcus mutans UA159 Map Core Streptococcal Competence Genes

In Streptococcus mutans, an oral colonizer associated with dental caries, development of competence for natural genetic transformation is triggered by either of two types of peptide pheromones, competence-stimulating peptides (CSPs) (18 amino acids [aa]) or SigX-inducing peptides (XIPs) (7 aa). Competence induced by CSP is a late response to the pheromone that requires the response regulator ComE and the XIP-encoding gene comS. XIP binds to ComR to allow expression of the alternative sigma factor SigX and the effector genes it controls. While these regulatory links are established, the precise set of effectors controlled by each regulator is poorly defined. To improve the definition of all three regulons, we used a high-resolution tiling array to map global changes in gene expression in the early and late phases of the CSP response. The early phase of the CSP response was limited to increased gene expression at four loci associated with bacteriocin production and immunity. In the late phase, upregulated regions expanded to a total of 29 loci, including comS and genes required for DNA uptake and recombination. The results indicate that the entire late response to CSP depends on the expression of comS and that the immediate transcriptional response to CSP, mediated by ComE, is restricted to just four bacteriocin-related loci. Comparison of the new data with published transcriptome data permitted the identification of all of the operons in each regulon: 4 for ComE, 2 for ComR, and 21 for SigX. Finally, a core set of 27 panstreptococcal competence genes was identified within the SigX regulon by comparison of transcriptome data from diverse streptococcal species. IMPORTANCES. mutans has the hard surfaces of the oral cavity as its natural habitat, where it depends on its ability to form biofilms in order to survive. The comprehensive identification of S. mutans regulons activated in response to peptide pheromones provides an important basis for understanding how S. mutans can transition from individual to social behavior. Our study placed 27 of the 29 transcripts activated during competence within three major regulons and revealed a core set of 27 panstreptococcal competence-activated genes within the SigX regulon.

HAT: a novel statistical approach to discover functional regions in the genome

Tiling arrays are useful for exploring local functions of regions of the genome in an unbiased fashion. The exact determination of those genomic regions based on tiling-array data, e.g., generated by means of hybridization with immunopreciptated DNA-fragments to the arrays is a challenge. Many different statistical methodologies have been developed to find biological relevant regions-of-interest (ROI) by using the quantitative signal intensity of each probe. We previously developed a method called Hypergeometric Analysis of Tiling arrays (HAT) for the analysis of tiling-array data, but it is developed such that it can also be used to study data derived by genome-wide deep sequencing approaches. Here we applied HAT to analyze two publicly available tiling-array data sets. After the detection of statistically significant ROI, these are often used in additional analysis for hypothesis testing. We therefore discuss, by using the results of the tiling-array experiment, pathway and motif analyses.

Comprehensive transcriptome analysis of the periodontopathogenic bacterium Porphyromonas gingivalis W83

High-density tiling microarray and RNA sequencing technologies were used to analyze the transcriptome of the periodontopathogenic bacterium Porphyromonas gingivalis. The compiled P. gingivalis transcriptome profiles were based on total RNA samples isolated from three different laboratory culturing conditions, and the strand-specific transcription profiles generated covered the entire genome, including both protein coding and noncoding regions. The transcription profiles revealed various operon structures, 5'- and 3'-end untranslated regions (UTRs), differential expression patterns, and many novel, not-yet-annotated transcripts within intergenic and antisense regions. Further transcriptome analysis identified the majority of the genes as being expressed within operons and most 5' and 3' ends to be protruding UTRs, of which several 3' UTRs were extended to overlap genes carried on the opposite/antisense strand. Extensive antisense RNAs were detected opposite most insertion sequence (IS) elements. Pairwise comparative analyses were also performed among transcriptome profiles of the three culture conditions, and differentially expressed genes and metabolic pathways were identified. With the growing realization that noncoding RNAs play important biological functions, the discovery of novel RNAs and the comprehensive transcriptome profiles compiled in this study may provide a foundation to further understand the gene regulation and virulence mechanisms in P. gingivalis. The transcriptome profiles can be viewed at and downloaded from the Microbial Transcriptome Database website, http://bioinformatics.forsyth.org/mtd.

Strand-specific transcriptome profiling with directly labeled RNA on genomic tiling microarrays

Background

With lower manufacturing cost, high spot density, and flexible probe design, genomic tiling microarrays are ideal for comprehensive transcriptome studies. Typically, transcriptome profiling using microarrays involves reverse transcription, which converts RNA to cDNA. The cDNA is then labeled and hybridized to the probes on the arrays, thus the RNA signals are detected indirectly. Reverse transcription is known to generate artifactual cDNA, in particular the synthesis of second-strand cDNA, leading to false discovery of antisense RNA. To address this issue, we have developed an effective method using RNA that is directly labeled, thus by-passing the cDNA generation. This paper describes this method and its application to the mapping of transcriptome profiles.

Results

RNA extracted from laboratory cultures of Porphyromonas gingivalis was fluorescently labeled with an alkylation reagent and hybridized directly to probes on genomic tiling microarrays specifically designed for this periodontal pathogen. The generated transcriptome profile was strand-specific and produced signals close to background level in most antisense regions of the genome. In contrast, high levels of signal were detected in the antisense regions when the hybridization was done with cDNA. Five antisense areas were tested with independent strand-specific RT-PCR and none to negligible amplification was detected, indicating that the strong antisense cDNA signals were experimental artifacts.

Conclusions

An efficient method was developed for mapping transcriptome profiles specific to both coding strands of a bacterial genome. This method chemically labels and uses extracted RNA directly in microarray hybridization. The generated transcriptome profile was free of cDNA artifactual signals. In addition, this method requires fewer processing steps and is potentially more sensitive in detecting small amount of RNA compared to conventional end-labeling methods due to the incorporation of more fluorescent molecules per RNA fragment.