Crystal structure of an ORFan protein (TM1622) from Thermotoga maritima at 1.75 A resolution reveals a fold similar to the Ran-binding protein Mog1p

Orphan Genes Shared by Pathogenic Genomes Are More Associated with Bacterial Pathogenicity

Recent pangenome analyses of numerous bacterial species have suggested that each genome of a single species may have a significant fraction of its gene content unique or shared by a very few genomes (i.e., ORFans). We selected nine bacterial genera, each containing at least five pathogenic and five nonpathogenic genomes, to compare their ORFans in relation to pathogenicity-related genes. Pathogens in these genera are known to cause a number of common and devastating human diseases such as pneumonia, diphtheria, melioidosis, and tuberculosis. Thus, they are worthy of in-depth systems microbiology investigations, including the comparative study of ORFans between pathogens and nonpathogens. We provide direct evidence to suggest that ORFans shared by more pathogens are more associated with pathogenicity-related genes and thus are more important targets for development of new diagnostic markers or therapeutic drugs for bacterial infectious diseases.

Orphan genes (also known as ORFans [i.e., orphan open reading frames]) are new genes that enable an organism to adapt to its specific living environment. Our focus in this study is to compare ORFans between pathogens (P) and nonpathogens (NP) of the same genus. Using the pangenome idea, we have identified 130,169 ORFans in nine bacterial genera (505 genomes) and classified these ORFans into four groups: (i) SS-ORFans (P), which are only found in a single pathogenic genome; (ii) SS-ORFans (NP), which are only found in a single nonpathogenic genome; (iii) PS-ORFans (P), which are found in multiple pathogenic genomes; and (iv) NS-ORFans (NP), which are found in multiple nonpathogenic genomes. Within the same genus, pathogens do not always have more genes, more ORFans, or more pathogenicity-related genes (PRGs)—including prophages, pathogenicity islands (PAIs), virulence factors (VFs), and horizontal gene transfers (HGTs)—than nonpathogens. Interestingly, in pathogens of the nine genera, the percentages of PS-ORFans are consistently higher than those of SS-ORFans, which is not true in nonpathogens. Similarly, in pathogens of the nine genera, the percentages of PS-ORFans matching the four types of PRGs are also always higher than those of SS-ORFans, but this is not true in nonpathogens. All of these findings suggest the greater importance of PS-ORFans for bacterial pathogenicity.

IMPORTANCE Recent pangenome analyses of numerous bacterial species have suggested that each genome of a single species may have a significant fraction of its gene content unique or shared by a very few genomes (i.e., ORFans). We selected nine bacterial genera, each containing at least five pathogenic and five nonpathogenic genomes, to compare their ORFans in relation to pathogenicity-related genes. Pathogens in these genera are known to cause a number of common and devastating human diseases such as pneumonia, diphtheria, melioidosis, and tuberculosis. Thus, they are worthy of in-depth systems microbiology investigations, including the comparative study of ORFans between pathogens and nonpathogens. We provide direct evidence to suggest that ORFans shared by more pathogens are more associated with pathogenicity-related genes and thus are more important targets for development of new diagnostic markers or therapeutic drugs for bacterial infectious diseases.

Structure of the sensor domain of Mycobacterium tuberculosis PknH receptor kinase reveals a conserved binding cleft

Since their discovery over 20 years ago, eukaryotic-like transmembrane receptor Ser/Thr protein kinases (STPKs) have been shown to play critical roles in the virulence, growth, persistence, and reactivation of many bacteria. Information regarding the signals transmitted by these proteins, however, remains scarce. To enhance understanding of the basis for STPK receptor signaling, we determined the 1.7-Å-resolution crystal structure of the extracellular sensor domain of the Mycobacterium tuberculosis receptor STPK, PknH (Rv1266c). The PknH sensor domain adopts an unanticipated fold containing two intramolecular disulfide bonds and a large hydrophobic and polar cleft. The residues lining the cleft and those surrounding the disulfide bonds are conserved. These results suggest that PknH binds a small-molecule ligand that signals by changing the location or quaternary structure of the kinase domain.

Decision-making in structure solution using Bayesian estimates of map quality: the PHENIX AutoSol wizard

Ten measures of experimental electron-density-map quality are examined and the skewness of electron density is found to be the best indicator of actual map quality. A Bayesian approach to estimating map quality is developed and used in the PHENIX AutoSol wizard to make decisions during automated structure solution.

Estimates of the quality of experimental maps are important in many stages of structure determination of macromolecules. Map quality is defined here as the correlation between a map and the corresponding map obtained using phases from the final refined model. Here, ten different measures of experimental map quality were examined using a set of 1359 maps calculated by re-analysis of 246 solved MAD, SAD and MIR data sets. A simple Bayesian approach to estimation of map quality from one or more measures is presented. It was found that a Bayesian estimator based on the skewness of the density values in an electron-density map is the most accurate of the ten individual Bayesian estimators of map quality examined, with a correlation between estimated and actual map quality of 0.90. A combination of the skewness of electron density with the local correlation of r.m.s. density gives a further improvement in estimating map quality, with an overall correlation coefficient of 0.92. The PHENIX AutoSol wizard carries out automated structure solution based on any combination of SAD, MAD, SIR or MIR data sets. The wizard is based on tools from the PHENIX package and uses the Bayesian estimates of map quality described here to choose the highest quality solutions after experimental phasing.

Structural features and the persistence of acquired proteins

ORFan genes can constitute a large fraction of a bacterial genome, but due to their lack of homologs, their functions have remained largely unexplored. To determine if particular features of ORFan-encoded proteins promote their presence in a genome, we analyzed properties of ORFans that originated over a broad evolutionary timescale. We also compared ORFan genes to another class of acquired genes, heterogeneous occurrence in prokaryotes (HOPs), which have homologs in other bacteria. A total of 54 ORFan and HOP genes selected from different phylogenetic depths in the Escherichia coli lineage were cloned, expressed, purified, and subjected to circular dichroism (CD) spectroscopy. A majority of genes could be expressed, but only 18 yielded sufficient soluble protein for spectral analysis. Of these, half were significantly alpha-helical, three were predominantly beta-sheet, and six were of intermediate/indeterminate structure. Although a higher proportion of HOPs yielded soluble proteins with resolvable secondary structures, ORFans resembled HOPs with regard to most of the other features tested. Overall, we found that those ORFan and HOP genes that have persisted in the E. coli lineage were more likely to encode soluble and folded proteins, more likely to display environmental modulation of their gene expression, and by extrapolation, are more likely to be functional.

Structure and ligand binding of the soluble domain of a Thermotoga maritima membrane protein of unknown function TM1634

As a part of the Joint Center for Structural Genomics (JCSG) biological targets, the structures of soluble domains of membrane proteins from Thermotoga maritima were pursued. Here, we report the crystal structure of the soluble domain of TM1634, a putative membrane protein of 128 residues (15.1 kDa) and unknown function. The soluble domain of TM1634 is an alpha-helical dimer that contains a single tetratrico peptide repeat (TPR) motif in each monomer where each motif is similar to that found in Tom20. The overall fold, however, is unique and a DALI search does not identify similar folds beyond the 38-residue TPR motif. Two different putative ligand binding sites, in which PEG200 and Co(2+) were located, were identified using crystallography and NMR, respectively.

Identification of novel suppressors for Mog1 implies its involvement in RNA metabolism, lipid metabolism and signal transduction

Mog1 is conserved from yeast to mammal, but its function is obscure. We isolated yeast genes that rescued a temperature-sensitive death of S. cerevisiae Scmog1Delta, and of S. pombe Spmog1(ts). Scmog1Delta was rescued by Opi3p, a phospholipid N-methyltransferase, in addition to S. cerevisiae Ran-homologue Gsp1p, and a RanGDP binding protein Ntf2p. On the other hand, Spmog1(ts) was rescued by Cid13 that is a poly (A) polymerase specific for suc22(+) mRNA encoding a subunit of ribonucleotide reductase, Ssp1 that is a protein kinase involved in stress response pathway, and Crp79 that is required for mRNA export, in addition to Spi1, S. pombe Ran-homologue, and Nxt2, S. pombe homologue of Ntf2p. Consistent with the identification of those suppressors, lack of ScMog1p dislocates Opi3p from the nuclear membrane and all of Spmog1(ts) showed the nuclear accumulation of mRNA. Furthermore, SpMog1 was co-precipitated with Nxt2 and Cid13.

The structure of RseB: a sensor in periplasmic stress response of E. coli

An elegant network of signal transduction has evolved in the bacterial cell envelope to respond to environmental stress. It is initiated by sensing unfavourable and harmful changes in the periplasm. The stress signal is then transmitted by a controlled degradation of the transmembrane anti-sigma-factor RseA that leads to the activation of the alternative sigma factor sigma(E). The periplasmic protein RseB exerts a crucial role in modulating the stability of RseA. RseB from Escherichia coli has been crystallized and crystal structures were determined at 2.4 A and at 2.8 A resolution. The protein forms a homodimer, with the monomer composed of two domains. The large domain resembles an unclosed beta-barrel that is structurally remarkably similar to a protein family capable of binding the lipid anchor of lipoproteins. The small C-terminal domain, connected to the large domain by a partially unstructured loop, is responsible for interaction with RseA. On the basis of the structure of RseB, we suggest that it acts as a sensor of periplasmic stress with a dual functionality: it detects mislocalized lipoproteins and propagates the signal to induce the sigma(E)-response.