The PreQ1-I riboswitch is a cis-acting element identified in bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 which regulates expression of genes involved in biosynthesis

Biosynthesis

Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...

 of the nucleoside

Nucleoside

Nucleosides are glycosylamines consisting of a nucleobase bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage...

 queuosine

Queuosine

Queuosine is a modified nucleoside that is present in certain tRNAs in bacteria and eukaryotes. Originally identified in E. coli, queuosine was found to occupy the first anticodon position of tRNAs for histidine, aspartic acid, asparagine and tyrosine. The first anticodon position pairs with the...

 from GTP

Guanosine triphosphate

Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...

. This RNA element known as a riboswitch

Riboswitch

In molecular biology, a riboswitch is a part of an mRNA molecule that can directly bind a small target molecule, and whose binding of the target affects the gene's activity. Thus, an mRNA that contains a riboswitch is directly involved in regulating its own activity, in response to the...

 binds preQ₁ (pre-queuosine₁), an intemediate in the queuosine pathway. Riboswitch function has been characterised in the bacterium Bacillus subtilis

Bacillus subtilis

Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...

, where the riboswitch is located in the leader of the ykvJKLM (queCDEF) operon

Operon

In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...

 which encodes four genes necessary for queuosine production . In this organism, PreQ₁ binding to the riboswitch aptamer

Aptamer

Aptamers are oligonucleic acid or peptide molecules that bind to a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool, but natural aptamers also exist in riboswitches. Aptamers can be used for both basic research and clinical purposes as...

 is thought to induce premature transcription

Transcription (genetics)

Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

 termination within the leader to down-regulate expression of these genes. The preQ₁ riboswitch is distinguished by its unusually small aptamer, compared to other riboswitches. Its atomic-resolution three dimensional structure was determined. PreQ1-II riboswitch

PreQ1-II riboswitch

PreQ1-II riboswitches form a class of riboswitches that specifically bind pre-queosine1 , a precursor of the modified nucleoside queuosine. They are found in certain species of Streptococcus and Lactococcus, and were originally identified as a conserved RNA secondary structure called the "COG4708...

es also bind preQ₁, but have an unrelated, distinct structure.