Geunu Bak

Recognition and discrimination of target mRNAs by Sib RNAs, a cis-encoded sRNA family

Five Sib antitoxin RNAs, members of a family of cis-encoded small regulatory RNAs (sRNAs) in Escherichia coli, repress their target mRNAs, which encode Ibs toxins. This target repression occurs only between cognate sRNA–mRNA pairs with an exception of ibsA. We performed co-transformation assays to assess the ability of SibC derivatives to repress ibsC expression, thereby revealing the regions of SibC that are essential for ibsC mRNA recognition. SibC has two target recognition domains, TRD1 and TRD2, which function independently. The target site for TRD1 is located within the ORF of ibsC, whereas the target site for TRD2 is located in the translational initiation region. The TRD1 sequence is sufficient to repress ibsC expression. In contrast, TRD2 requires a specific structure in addition to the recognition sequence. An in vitro structural probing analysis showed that the initial interactions at these two recognition sites allowed base-pairing to progress into the flanking sequences. Displacement of the TRD1 and TRD2 domains of SibC by the corresponding domains of SibD changed the target specificity of SibC from ibsC to ibsD, suggesting that these two elements modulate the cognate target recognition of each Sib RNA by discriminating among non-cognate ibs mRNAs.

SET7/9 Methylation of the Pluripotency Factor LIN28A Is a Nucleolar Localization Mechanism that Blocks let-7 Biogenesis in Human ESCs

LIN28-mediated processing of the microRNA (miRNA) let-7 has emerged as a multilevel program that controls self-renewal in embryonic stem cells. LIN28A is believed to act primarily in the cytoplasm together with TUT4/7 to prevent final maturation of let-7 by Dicer, whereas LIN28B has been suggested to preferentially act on nuclear processing of let-7. Here, we find that SET7/9 monomethylation in a putative nucleolar localization region of LIN28A increases its nuclear retention and protein stability. In the nucleoli of human embryonic stem cells, methylated LIN28A sequesters pri-let-7 and blocks its processing independently of TUT4/7. The nuclear form of LIN28A regulates transcriptional changes in MYC-pathway targets, thereby maintaining stemness programs and inhibiting expression of early lineage-specific markers. These findings provide insight into the molecular mechanism underlying the posttranslational methylation of nuclear LIN28A and its ability to modulate pluripotency by repressing let-7 miRNA expression in human embryonic stem cells.

Systematic analysis of the role of bacterial Hfq-interacting sRNAs in the response to antibiotics

OBJECTIVES To systematically analyse the interplay between the expression of Hfq-associated small non-coding RNAs (sRNAs) and antibiotic susceptibility in Gram-negative bacteria. METHODS To identify the roles of sRNAs in the antibiotic susceptibility of Escherichia coli and Salmonella species, susceptibility tests, growth analyses and viability assays were performed using E. coli Hfq-associated sRNAs from overexpression libraries. Prediction, susceptibility testing of gene knockouts and expression analysis of target genes under conditions of sRNA overexpression or knockout were performed to identify candidate targets for modulating antibiotic susceptibility. RESULTS The susceptibilities of E. coli strains overexpressing each of the 26 known Hfq-dependent sRNAs to major classes of antibiotics were determined. Induced expression of 17 sRNAs modulated the susceptibility of E. coli to antibiotics. Among them, four sRNA knockout strains partially or completely reversed susceptibility phenotypes of sRNA overexpression. The phenotype of OxyS, RseX or MicF was not entirely dependent on the presence of Hfq protein, in contrast to the dependency of previously characterized roles. The function of eight of nine sRNAs was found to be conserved in the response to antibiotics in Salmonella. Some MicF- or RyeB-mediated cellular target genes and pathways that may be important for the regulation of antibiotic susceptibility were identified. Finally, the overexpression of RyeB potentiated the efficacy of levofloxacin against MDR strains. CONCLUSIONS Our data indicate that Hfq-associated sRNAs potentially enable bacteria to adapt to antibiotic challenges via multifaceted approaches. Therefore, sRNA-based applications will form a new antibiotic arsenal for combating the rise in antibiotic resistance.

Exploring sRNA-mediated gene silencing mechanisms using artificial small RNAs derived from a natural RNA scaffold in Escherichia coli

An artificial small RNA (afsRNA) scaffold was designed from an Escherichia coli sRNA, SibC. Using the lacZ reporter system, the gene silencing effects of afsRNAs were examined to explore the sRNA-mediated gene-silencing mechanisms in E. coli. Substitution of the original target recognition sequence with a new sequence recognizing lacZ mRNA led to effective reduction of lacZ gene expression. Single-strandedness of the target recognition sequences in the scaffold was essential for effective gene silencing. The target recognition sequence was shortened to 10 nt without significant loss of gene silencing, although this minimal length was limited to a specific target mRNA sequence. In cases where afsRNAs had mismatched (forming internal loops) or unmatched (forming bulges) regions in the middle of the target recognition sequence, internal loop-forming afsRNAs were more effective in gene silencing than those that formed bulges. Unexpectedly, gene silencing by afsRNA was not decreased but increased on hfq disruption in E. coli, particularly when interactions between afsRNA and mRNA were weak, suggesting that Hfq is possibly involved in destabilization of the RNA–RNA duplex, rather than enhancement of base pairing.

Roles of rpoS‐activating small RNAs in pathways leading to acid resistance of Escherichia coli

Escherichia coli and related enteric bacteria can survive under extreme acid stress condition at least for several hours. RpoS is a key factor for acid stress management in many enterobacteria. Although three rpoS‐activating sRNAs, DsrA, RprA, and ArcZ, have been identified in E. coli, it remains unclear how these small RNA molecules participate in pathways leading to acid resistance (AR). Here, we showed that overexpression of ArcZ, DsrA, or RprA enhances AR in a RpoS‐dependent manner. Mutant strains with deletion of any of three sRNA genes showed lowered AR, and deleting all three sRNA genes led to more severe defects in protecting against acid stress. Overexpression of any of the three sRNAs fully rescued the acid tolerance defects of the mutant strain lacking all three genes, suggesting that all three sRNAs perform the same function in activating RpoS required for AR. Notably, acid stress led to the induction of DsrA and RprA but not ArcZ.

Identification of novel sRNAs involved in biofilm formation, motility, and fimbriae formation in Escherichia coli.

Bacterial small RNAs (sRNAs) are known regulators in many physiological processes. In Escherichia coli, a large number of sRNAs have been predicted, among which only about a hundred are experimentally validated. Despite considerable research, the majority of their functions remain uncovered. Therefore, collective analysis of the roles of sRNAs in specific cellular processes may provide an effective approach to identify their functions. Here, we constructed a collection of plasmids overexpressing 99 individual sRNAs, and analyzed their effects on biofilm formation and related phenotypes. Thirty-three sRNAs significantly affecting these cellular processes were identified. No consistent correlations were observed, except that all five sRNAs suppressing type I fimbriae inhibited biofilm formation. Interestingly, IS118, yet to be characterized, suppressed all the processes. Our data not only reveal potentially critical functions of individual sRNAs in biofilm formation and other phenotypes but also highlight the unexpected complexity of sRNA-mediated metabolic pathways leading to these processes.

Regulation of transcription from two ssrS promoters in 6S RNA biogenesis

AbstractssrS-encoded 6S RNA is an abundant noncoding RNA that binds σ70-RNA polymerase and regulates expression at a subset of promoters in Escherichia coli. It is transcribed from two tandem promoters, ssrS P1 and ssrS P2. Regulation of transcription from two ssrS promoters in 6S RNA biogenesis was examined. Both P1 and P2 were growth phase-dependently regulated. Depletion of 6S RNA had no effect on growth-phase-dependent transcription from either promoter, whereas overexpression of 6S RNA increased P1 transcription and decreased P2 transcription, suggesting that transcription from P1 and P2 is subject to feedback activation and feedback inhibition, respectively. This feedback regulation disappeared in Δfis strains, supporting involvement of Fis in this process. The differential feedback regulation may provide a means for maintaining appropriate cellular concentrations of 6S RNA.

Regulation of BC200 RNA‐mediated translation inhibition by hnRNP E1 and E2

The long noncoding RNA BC200 (brain cytoplasmic RNA, 200 nucleotides) acts as a translational modulator of local protein synthesis at dendrites. BC200 RNA has been shown to inhibit translation in vitro, but it remains unknown how this translation inhibition might be controlled in a cell. Here, we performed yeast three‐hybrid screening and identified hnRNP E1 and hnRNP E2 as BC200 RNA‐interacting proteins. We found that: these hnRNA proteins could restore BC200 RNA‐inhibited translation; BC200 RNA interacts with hnRNP E1 and E2 mainly through its unique 3′ C‐rich domain; and the RNA binding specificities and modes of the two proteins differed somewhat. Our results offer new insights into the regulation of BC200 RNA‐mediated translation inhibition.

Electrophoretic mobility shift assay of RNA-RNA complexes.

A simple, rapid, and sensitive electrophoretic mobility shift assay (EMSA) can be successfully used to analyze RNA-RNA interactions. The EMSA of RNA-RNA complexes can be further used to evaluate the specificity of interactions using competitor RNAs in combination with their mutated versions or nonspecific RNAs, such as yeast tRNA. RNA is simply prepared by in vitro transcription from PCR product templates. Detailed experimental descriptions for EMSA-based analysis of specific RNA-RNA interactions between Sib RNAs and ibs mRNAs as a representative example are presented.

The CnuK9E H-NS complex antagonizes DNA binding of DicA and leads to temperature-dependent filamentous growth in E. coli.

Cnu (an OriC-binding nucleoid protein) associates with H-NS. A variant of Cnu was identified as a key factor for filamentous growth of a wild-type Escherichia coli strain at 37°C. This variant (CnuK9E) bears a substitution of a lysine to glutamic acid, causing a charge reversal in the first helix. The temperature-dependent filamentous growth of E. coli bearing CnuK9E could be reversed by either lowering the temperature to 25°C or lowering the CnuK9E concentration in the cell. Gene expression analysis suggested that downregulation of dicA by CnuK9E causes a burst of dicB transcription, which, in turn, elicits filamentous growth. In vivo assays indicated that DicA transcriptionally activates its own gene, by binding to its operator in a temperature-dependent manner. The antagonizing effect of CnuK9E with H-NS on DNA-binding activity of DicA was stronger at 37°C, presumably due to the lower operator binding of DicA at 37°C. These data suggest that the temperature-dependent negative effect of CnuK9E on DicA binding plays a major role in filamentous growth. The C-terminus of DicA shows significant amino acid sequence similarity to the DNA-binding domains of RovA and SlyA, regulators of pathogenic genes in Yersinia and Salmonella, respectively, which also show better DNA-binding activity at 25°C.