Discovery of Unannotated Small Open Reading Frames in Streptococcus pneumoniae D39 Involved in Quorum Sensing and Virulence Using Ribosome Profiling

Biological puzzles solved by using Streptococcus pneumoniae: a historical review of the pneumococcal studies that have impacted medicine and shaped molecular bacteriology

The major human pathogen Streptococcus pneumoniae has been the subject of intensive clinical and basic scientific study for over 140 years. In multiple instances, these efforts have resulted in major breakthroughs in our understanding of basic biological principles as well as fundamental tenets of bacterial pathogenesis, immunology, vaccinology, and genetics. Discoveries made with S. pneumoniae have led to multiple major public health victories that have saved the lives of millions. Studies on S. pneumoniae continue today, where this bacterium is being used to dissect the impact of the host on disease processes, as a powerful cell biology model, and to better understand the consequence of human actions on commensal bacteria at the population level. Herein we review the major findings, i.e., puzzle pieces, made with S. pneumoniae and how, over the years, they have come together to shape our understanding of this bacterium's biology and the practice of medicine and modern molecular biology.

Microproteins-Discovery, structure, and function

Advances in proteogenomic technologies have revealed hundreds to thousands of translated small open reading frames (sORFs) that encode microproteins in genomes across evolutionary space. While many microproteins have now been shown to play critical roles in biology and human disease, a majority of recently identified microproteins have little or no experimental evidence regarding their functionality. Computational tools have some limitations for analysis of short, poorly conserved microprotein sequences, so additional approaches are needed to determine the role of each member of this recently discovered polypeptide class. A currently underexplored avenue in the study of microproteins is structure prediction and determination, which delivers a depth of functional information. In this review, we provide a brief overview of microprotein discovery methods, then examine examples of microprotein structures (and, conversely, intrinsic disorder) that have been experimentally determined using crystallography, cryo-electron microscopy, and NMR, which provide insight into their molecular functions and mechanisms. Additionally, we discuss examples of predicted microprotein structures that have provided insight or context regarding their function. Analysis of microprotein structure at the angstrom level, and confirmation of predicted structures, therefore, has potential to identify translated microproteins that are of biological importance and to provide molecular mechanism for their in vivo roles.

The power of the small: the underestimated role of small proteins in bacterial and archaeal physiology

Small proteins encoded by small open-reading frames (sORFs) (≤70 aa) were overlooked for decades due to methodological reasons and are thus often missing in genome annotations. Novel detection methods such as ribosome profiling (Ribo-Seq) and mass spectrometry optimized for small proteins (peptidomics) have opened up a new field of interest and several catalogs of small proteins in bacteria and archaea have been recently reported. Many translated sORFs have been discovered in genomic locations previously thought to be noncoding, such as 5' or 3' untranslated regions or well-studied regulatory small RNAs (sRNAs). Even within longer ORFs, additional functional sORFs have been detected. Today, only a small proportion is characterized, but those small proteins indicate important and diverse functions in cellular physiology. Here, we summarize recently characterized small proteins involved in microbial metabolism.

Exposing the small protein load of bacterial life

The ever-growing repertoire of genomic techniques continues to expand our understanding of the true diversity and richness of prokaryotic genomes. Riboproteogenomics laid the foundation for dynamic studies of previously overlooked genomic elements. Most strikingly, bacterial genomes were revealed to harbor robust repertoires of small open reading frames (sORFs) encoding a diverse and broadly expressed range of small proteins, or sORF-encoded polypeptides (SEPs). In recent years, continuous efforts led to great improvements in the annotation and characterization of such proteins, yet many challenges remain to fully comprehend the pervasive nature of small proteins and their impact on bacterial biology. In this work, we review the recent developments in the dynamic field of bacterial genome reannotation, catalog the important biological roles carried out by small proteins and identify challenges obstructing the way to full understanding of these elusive proteins.

Identification and Characterization of Negative Regulators of Rgg1518 Quorum Sensing in Streptococcus pneumoniae

Streptococcus pneumoniae is an agent of otitis media, septicemia, and meningitis and remains the leading cause of community-acquired pneumonia regardless of vaccine use. Of the various strategies that S. pneumoniae takes to enhance its potential to colonize the human host, quorum sensing (QS) is an intercellular communication process that provides coordination of gene expression at a community level. Numerous putative QS systems are identifiable in the S. pneumoniae genome, but their gene-regulatory activities and contributions to fitness have yet to be fully evaluated. To contribute to assessing regulatory activities of rgg paralogs present in the D39 genome, we conducted transcriptomic analysis of mutants of six QS regulators. Our results find evidence that at least four QS regulators impact the expression of a polycistronic operon (encompassing genes spd_1517 to spd_1513) that is directly controlled by the Rgg/SHP1518 QS system. As an approach to unravel the convergent regulation placed on the spd_1513-1517 operon, we deployed transposon mutagenesis screening in search of upstream regulators of the Rgg/SHP1518 QS system. The screen identified two types of insertion mutants that result in increased activity of Rgg1518-dependent transcription, one type being where the transposon inserted into pepO, an annotated endopeptidase, and the other type being insertions in spxB, a pyruvate oxidase. We demonstrate that pneumococcal PepO degrades SHP1518 to prevent activation of Rgg/SHP1518 QS. Moreover, the glutamic acid residue in the conserved "HExxH" domain is indispensable for the catalytic function of PepO. Finally, we confirmed the metalloendopeptidase property of PepO, which requires zinc ions, but not other ions, to facilitate peptidyl hydrolysis. IMPORTANCE Streptococcus pneumoniae uses quorum sensing to communicate and regulate virulence. In our study, we focused on one Rgg quorum sensing system (Rgg/SHP1518) and found that multiple other Rgg regulators also control it. We further identified two enzymes that inhibit Rgg/SHP1518 signaling and revealed and validated one enzyme's mechanisms for breaking down quorum sensing signaling molecules. Our findings shed light on the complex regulatory network of quorum sensing in Streptococcus pneumoniae.

The ComRS-SigX Pathway Regulates Natural Transformation in Streptococcus ferus

The ability to take up and incorporate foreign DNA via natural transformation is a well-known characteristic of some species of Streptococcus, and is a mechanism that rapidly allows for the acquisition of antibacterial resistance. Here, we describe that the understudied species Streptococcus ferus is also capable of natural transformation and uses a system analogous to that identified in Streptococcus mutans. S. mutans natural transformation is under the control of the alternative sigma factor sigX (also known as comX), whose expression is induced by two types of peptide signals: CSP (competence stimulating peptide, encoded by comC) and XIP (sigX-inducing peptide, encoded by comS). These systems induce competence via either the two-component signal-transduction system ComDE or the RRNPP transcriptional regulator ComR, respectively. Protein and nucleotide homology searches identified putative orthologs of comRS and sigX in S. ferus, but not homologs of S. mutans blpRH (also known as comDE). We demonstrate that natural transformation in S. ferus is induced by a small, double-tryptophan containing sigX-inducing peptide (XIP), akin to that of S. mutans, and requires the presence of the comR and sigX orthologs for efficient transformation. Additionally, we find that natural transformation is induced in S. ferus by both the native XIP and the XIP variant of S. mutans, implying that cross talk between the two species is possible. This process has been harnessed to construct gene deletions in S. ferus and provides a method to genetically manipulate this understudied species. IMPORTANCE Natural transformation is the process by which bacteria take up DNA and allows for acquisition of new genetic traits, including those involved in antibiotic resistance. This study demonstrates that the understudied species Streptococcus ferus is capable of natural transformation using a peptide-pheromone system like that previously identified in Streptococcus mutans and provides a framework for future studies concerning this organism.

The ComRS-SigX pathway regulates natural transformation in Streptococcus ferus

The ability to take up and incorporate foreign DNA via natural transformation is a well-known characteristic of some species of Streptococcus, and is a mechanism that rapidly allows for the acquisition of antibacterial resistance. Here, we describe that the understudied species Streptococcus ferus is also capable of natural transformation and uses a system analogous to that identified in Streptococcus mutans . S. mutans natural transformation is under the control of the alternative sigma factor sigX (also known as comX ), whose expression is induced by two types of peptide signals: CSP ( c ompetence s timulating p eptide, encoded by comC ) and XIP ( sig X -inducing p eptide, encoded by comS ). These systems induce competence via either the two-component signal-transduction system ComDE or the RRNPP transcriptional regulator ComR, respectively. Protein and nucleotide homology searches identified putative orthologs of comRS and sigX in S. ferus , but not homologs of S. mutans blpRH (also known as comDE ). We demonstrate that natural transformation in S. ferus is induced by a small, double-tryptophan containing competence-inducing peptide (XIP), akin to that of S. mutans , and requires the presence of the comR and sigX orthologs for efficient transformation. Additionally, we find that natural transformation is induced in S. ferus by both the native XIP and the XIP variant of S. mutans , implying that crosstalk between the two species is possible. This process has been harnessed to construct gene deletions in S. ferus and provides a method to genetically manipulate this understudied species.

IMPORTANCE

Natural transformation is the process by which bacteria take up DNA and allows for acquisition of new genetic traits, including those involved in antibiotic resistance. This study demonstrates that the understudied species Streptococcus ferus is capable of natural transformation using a peptide-pheromone system like that previously identified in Streptococcus mutans and provides a framework for future studies concerning this organism.