Noncanonical usage of stop codons in ciliates expands proteins with structurally flexible Q-rich motifs

Serine(S)/threonine(T)-glutamine(Q) cluster domains (SCDs), polyglutamine (polyQ) tracts and polyglutamine/asparagine (polyQ/N) tracts are Q-rich motifs found in many proteins. SCDs often are intrinsically disordered regions that mediate protein phosphorylation and protein-protein interactions. PolyQ and polyQ/N tracts are structurally flexible sequences that trigger protein aggregation. We report that due to their high percentages of STQ or STQN amino acid content, four SCDs and three prion-causing Q/N-rich motifs of yeast proteins possess autonomous protein expression-enhancing activities. Since these Q-rich motifs can endow proteins with structural and functional plasticity, we suggest that they represent useful toolkits for evolutionary novelty. Comparative Gene Ontology (GO) analyses of the near-complete proteomes of 26 representative model eukaryotes reveal that Q-rich motifs prevail in proteins involved in specialized biological processes, including Saccharomyces cerevisiae RNA-mediated transposition and pseudohyphal growth, Candida albicans filamentous growth, ciliate peptidyl-glutamic acid modification and microtubule-based movement, Tetrahymena thermophila xylan catabolism and meiosis, Dictyostelium discoideum development and sexual cycles, Plasmodium falciparum infection, and the nervous systems of Drosophila melanogaster, Mus musculus and Homo sapiens. We also show that Q-rich-motif proteins are expanded massively in 10 ciliates with reassigned TAAQ and TAGQ codons. Notably, the usage frequency of CAGQ is much lower in ciliates with reassigned TAAQ and TAGQ codons than in organisms with expanded and unstable Q runs (e.g. D. melanogaster and H. sapiens), indicating that the use of noncanonical stop codons in ciliates may have coevolved with codon usage biases to avoid triplet repeat disorders mediated by CAG/GTC replication slippage.

Genomic Instability of G-Quadruplex Sequences in Escherichia coli: Roles of DinG, RecG, and RecQ Helicases

Guanine-rich DNA can fold into highly stable four-stranded DNA structures called G-quadruplexes (G4). Originally identified in sequences from telomeres and oncogene promoters, they can alter DNA metabolism. Indeed, G4-forming sequences represent obstacles for the DNA polymerase, with important consequences for cell life as they may lead to genomic instability. To understand their role in bacterial genomic instability, different G-quadruplex-forming repeats were cloned into an Escherichia coli genetic system that reports frameshifts and complete or partial deletions of the repeat when the G-tract comprises either the leading or lagging template strand during replication. These repeats formed stable G-quadruplexes in single-stranded DNA but not naturally supercoiled double-stranded DNA. Nevertheless, transcription promoted G-quadruplex formation in the resulting R-loop for (G3T)4 and (G3T)8 repeats. Depending on genetic background and sequence propensity for structure formation, mutation rates varied by five orders of magnitude. Furthermore, while in vitro approaches have shown that bacterial helicases can resolve G4, it is still unclear whether G4 unwinding is important in vivo. Here, we show that a mutation in recG decreased mutation rates, while deficiencies in the structure-specific helicases DinG and RecQ increased mutation rates. These results suggest that G-quadruplex formation promotes genetic instability in bacteria and that helicases play an important role in controlling this process in vivo.

Tandem Repeat Generation and Novel Isothermal Amplification Based on Nonspecific Tailing and Replication Slippage

BACKGROUND

Isothermal amplification is considered to be one of the most promising tools for point-of-care testing molecular diagnosis. However, its clinical application is severely hindered by nonspecific amplification. Thus, it is important to investigate the exact mechanism of nonspecific amplification and develop a high-specific isothermal amplification assay.

METHODS

Four sets of primer pairs were incubated with Bst DNA polymerase to produce nonspecific amplification. Gel electrophoresis, DNA sequencing, and sequence function analysis were used to investigate the mechanism of nonspecific product generation, which was discovered to be nonspecific tailing and replication slippage mediated tandem repeats generation (NT&RS). Using this knowledge, a novel isothermal amplification technology, bridging primer assisted slippage isothermal amplification (BASIS), was developed.

RESULTS

During NT&RS, the Bst DNA polymerase triggers nonspecific tailing on the 3'-ends of DNAs, thereby producing sticky-end DNAs over time. The hybridization and extension between these sticky DNAs generate repetitive DNAs, which can trigger self-extension via replication slippage, thereby leading to nonspecific tandem repeats (TRs) generation and nonspecific amplification. Based on the NT&RS, we developed the BASIS assay. The BASIS is carried out by using a well-designed bridging primer, which can form hybrids with primer-based amplicons, thereby generating specific repetitive DNA and triggering specific amplification. The BASIS can detect 10 copies of target DNA, resist interfering DNA disruption, and provide genotyping ability, thereby offering 100% accuracy for type 16 human papillomavirus detection.

CONCLUSION

We discovered the mechanism for Bst-mediated nonspecific TRs generation and developed a novel isothermal amplification assay (BASIS), which can detect nucleic acids with high sensitivity and specificity.

Marsupial genome analysis suggests that satellite DNA formation from walb endogenous retrovirus is an event specific to the red-necked wallaby

We recently identified walbRep, a satellite DNA residing in the genome of the red-necked wallaby Notamacropus rufogriseus. It originates from the walb endogenous retrovirus and is organized in a manner in which the provirus structure is retained. The walbRep repeat units feature an average pairwise nucleotide identity as high as 99.5%, raising the possibility of a recent origin. The tammar wallaby N. eugenii is a species estimated to have diverged from the red-necked wallaby 2-3 million years ago. In PCR analyses of these two and other related species, walbRep-specific fragment amplification was observed only in the red-necked wallaby. Sequence database searches for the tammar wallaby resulted in sequence alignment lists that were sufficiently powerful to exclude the possibility of walbRep existence. These results suggested that the walbRep formation occurred in the red-necked wallaby lineage after its divergence from the tammar wallaby lineage, thus in a time span of maximum 3 million years.

Rapid emergence of African swine fever virus variants with different numbers of a tandem repeat sequence in South Korea

African swine fever virus variants with different numbers of a 10‐bp tandem repeat were isolated in South Korea soon after being identified in wild boar. The short emergence periods and sympatric distributions within a narrow geographical region suggest that the variants were sporadically generated in the pre‐existing viral population.

The Pneumococcal Serotype 15C Capsule Is Partially O-Acetylated and Allows for Limited Evasion of 23-Valent Pneumococcal Polysaccharide Vaccine-Elicited Anti-Serotype 15B Antibodies

As a species, Streptococcus pneumoniae (the pneumococcus) utilizes a diverse array of capsular polysaccharides to evade the host. In contrast to large variations in sugar composition and linkage formation, O-acetylation is a subtle capsular modification that nonetheless has a large impact on capsular shielding and recognition of the capsule by vaccine-elicited antibodies. Serotype 15B, which is included in the 23-valent pneumococcal polysaccharide vaccine (PPV23), carries the putative O-acetyltransferase gene wciZ The coding sequence of wciZ contains eight consecutive TA repeats [(TA)₈]. Replication slippage is thought to result in the addition or loss of TA repeats, subsequently causing frameshift and truncation of WciZ to yield a nonacetylated serotype, 15C. Using sensitive serological tools, we show that serotype 15C isolates whose wciZ contains seven or nine TA repeats retain partial O-acetylation, while serotype 15C isolates whose wciZ contains six TA repeats have barely detectable O-acetylation. We confirmed by inhibition enzyme-linked immunosorbent assay that (TA)₇ serotype 15C is ∼0.1% as acetylated as serotype 15B, while serotype 15X is nonacetylated. To eliminate the impact of genetic background, we created isogenic serotype 15B, (TA)₇ serotype 15C, and 15BΔwciZ (15X) strains and found that reduction or absence of WciZ-mediated O-acetylation did not affect capsular shielding from phagocytes, biofilm formation, adhesion to nasopharyngeal cells, desiccation tolerance, or murine colonization. Sera from PPV23-immunized persons opsonized serotype 15B significantly but only slightly better than serotypes 15C and 15X; thus, PPV23 may not result in expansion of serotype 15C.