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Upon nitrogen deficiency, some filamentous cyanobacteria differentiate specialized cells, called heterocysts, devoted to N2 fixation. Heterocysts appear regularly spaced along the filaments and exhibit structural and metabolic adaptations, such as loss of photosynthetic CO2 fixation or increased respiration, to provide a proper microaerobic environment for its specialized function. Heterocyst development is under transcriptional control of the global nitrogen regulator NtcA and the specific regulator HetR. Transcription of a large number of genes is induced or repressed upon nitrogen deficiency specifically in cells undergoing differentiation. In recent years, the HetR regulon has been described to include heterocyst-specific trans-acting small RNAs and antisense RNAs (asRNAs), suggesting that there is an additional layer of post-transcriptional regulation involved in heterocyst development. Here, we characterize in the cyanobacterium Nostoc (Anabaena) sp. PCC 7120 an asRNA, that we call as_glpX, transcribed within the glpX gene encoding the Calvin cycle bifunctional enzyme sedoheptulose-1,7- bisphosphatase/fructose-1,6-bisphosphatase (SBPase). Transcription of as_glpX is restricted to heterocysts and is induced very early during the process of differentiation. Expression of as_glpX RNA promotes the cleavage of the glpX mRNA by RNase III, resulting in a reduced amount of SBPase. Therefore, the early expression of this asRNA could contribute to the quick shut-down of CO2 fixation in those cells in the filament that are undergoing differentiation into heterocysts. In summary, as_glpX is the first naturally occurring asRNA shown to rapidly and dynamically regulate metabolic transformation in Nostoc heterocysts. The use of antisense transcripts to manipulate gene expression specifically in heterocysts could became a useful tool for metabolic engineering in cyanobacteria.

Temperature influences the structural and functional properties of cellular components, necessitating stress responses to restore homeostasis following temperature shift. Whereas the circuitry controlling the heat shock response is well understood, that controlling the E. coli cold shock adaptation program is not. We found that during the growth arrest phase (acclimation) that follows shift to low temperature, protein synthesis increases, and open reading frame (ORF)-wide mRNA secondary structure decreases. To identify the regulatory system controlling this process, we screened for players required for increased translation. We identified a two-member mRNA surveillance system that enables recovery of translation during acclimation: RNase R assures appropriate mRNA degradation and the Csps dynamically adjust mRNA secondary structure to globally modulate protein expression level. An autoregulatory switch in which Csps tune their own expression to cellular demand enables dynamic control of global translation. The universality of Csps in bacteria suggests broad utilization of this control mechanism.

Small RNAs (sRNAs), including microRNAs and endogenous siRNAs (endo-siRNAs), regulate most important biologic processes in eukaryotes, such as cell division and differentiation. Although sRNAs have been extensively studied in various eukaryotes, the role of sRNAs in the early emergence of eukaryotes is unclear. To address these questions, we deep sequenced the sRNA transcriptome of four different stages in the differentiation of Giardia lamblia, one of the most primitive eukaryotes. We identified a large number of endo-siRNAs in this fascinating parasitic protozoan and found that they were produced from live telomeric retrotransposons and three genomic regions (i.e., endo-siRNA generating regions [eSGRs]). eSGR-derived endo-siRNAs were proven to target mRNAs in trans. Gradual up-regulation of endo-siRNAs in the differentiation of Giardia suggested that they might be involved in the regulation of this process. This hypothesis was supported by the impairment of the differentiation ability of Giardia when GLDICER, essential for the biogenesis of endo-siRNAs, was knocked down. Endo-siRNAs are not the only sRNA regulators in Giardia differentiation, because a great number of tRNAs-derived sRNAs showed more dramatic expression changes than endo-siRNAs in this process. We totally identified five novel kinds of tRNAs-derived sRNAs and found that the biogenesis in four of them might be correlated with that of stress-induced tRNA-derived RNA (sitRNA), which was discovered in our previous studies. Our studies reveal an unexpected complex panorama of sRNA in G. lamblia and shed light on the origin and functional evolution of eukaryotic sRNAs.

Historically, the ribosome has been viewed as a ribozyme with constitutive rather than regulatory capacity in mRNA translation. However, emerging studies reveal that ribosome activity may be highly regulated to impart a new layer of specificity in control of gene expression and organismal biology. Heterogeneity in the composition of the ribosome may generate variations to the translation machinery that significantly impact on how the genomic template is translated into functional proteins. Studies in our laboratory have shown that ribosomal protein RPL38 confers specificity to the ribosome and is responsible for specifically translating a subset of Homeobox (Hox) mRNAs to regulate the mammalian body plan. In Rpl38 mutant embryos, global protein synthesis is unchanged while the translation of 8 of the 39 Hox mRNAs is specifically affected. Unexpectedly, Rpl38 transcripts are highly enriched in a tissue specific manner, most notably in somites and within the neural tube. Together, these studies led us to propose that ribosome in distinct regions of the embryo may be highly specialized to exert transcript-specific translational control. However how transcripts are recognized and selected by RPL38 remains unknown.Here I uncover a unique set of RNA regulons embedded in the 5'UTRs of Hox genes which are critical for ribosome-directed control of their spatiotemporal expression patterns. These structured RNA elements, resembling Viral Internal Ribosome Entry Sites (IRES), are positioned close to the AUG of subsets of Hox mRNA to interact with ribosomal proteins and directly recruit the ribosome. Using Hoxa9 as a model, I find that the IRES element folds into a stable structure in vitro and can recruit ribosomes directly to the mRNA. By generating the first targeted knock-out of an IRES element in the mouse, I show that the Hoxa9 transcript, despite numerous layers of transcriptional control, requires an IRES-element for accurate spatiotemporal expression and axial skeletal

Our ability to efficiently and predictably program cells is central to the fields of bioengineering and synthetic biology. Once thought to be a passive carrier of genetic information, RNA is now more appreciated as the main organizer of cellular networks. To harness the unique abilities of RNA molecules for programming cells, we show here how to rationally design novel synthetic RNA elements to recapitulate the functions of natural noncoding RNAs (ncRNAs), and how to assemble these synthetic elements into higher-order biological systems. To create synthetic RNA elements, we start with two primary types of ncRNA-mediated natural systems. Both modulate RNA-level regulatory signals encoded in the 5' untranslated region, and are mediated by ncRNAs. In the first system the ncRNA represses transcription elongation, whereas in the second system the ncRNA inhibits translation initiation. To create orthogonal RNA elements that work independently in the same cell, we systematically modify the RNA-RNA interaction in the natural systems. Our characterization results in families of orthogonally acting RNA elements for both transcription and translation controls. Furthermore, we develop mathematical thermodynamic models to predict new RNA elements in silico for translation controls. To engineer synthetic RNAs to sense and integrate cellular signals, we design allosteric RNA chimera molecules by fusing ncRNAs to RNA aptamers. We demonstrate the design principles for creating such chimeric RNA molecules that can sense proteins or small molecules and control transcription or translation. We show that the design strategy is modular, which allows us to reconfigure different ncRNA mutants and RNA aptamers to engineer orthogonal RNA chimeras that respond to different ligands and regulate different gene targets. We further show that multiple RNA chimeras allow logical integration of molecular signals in the same cell for cellular information processing. We assemble multiple synthetic RNA