In molecular biology, the PyrC leader is a
''cis''-regulatory RNA element found at the
5' of the PyrC
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
in
Enterobacteria
Enterobacteriaceae is a large family of Gram-negative bacteria. It was first proposed by Rahn in 1936, and now includes over 30 genera and more than 100 species. Its classification above the level of family is still a subject of debate, but on ...
. The PyrC gene encodes
Dihydroorotase
Dihydroorotase (, ''carbamoylaspartic dehydrase'', ''dihydroorotate hydrolase'') is an enzyme which converts carbamoyl aspartic acid into 4,5-dihydroorotic acid in the biosynthesis of pyrimidines. It forms a multifunctional enzyme with carbamoyl ...
, an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
involved in
pyrimidine biosynthesis
Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The other ...
.
The PyrC leader
regulates expression of PyrC.
Translation
Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
initiation can occur at four different sites within this leader sequence, under high
CTP conditions the translation initiation site is upstream of that used under low CTP conditions, additional
cytosine
Cytosine () ( symbol C or Cyt) is one of the four nucleobases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an am ...
residues are incorporated into the mRNA resulting in the formation of an
RNA hairpin. This hairpin blocks
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
-binding at the
Shine-Dalgarno sequence, and therefore blocks expression of PyrC. Under low CTP conditions the initiation site is further downstream and does not result in hairpin formation, so the ribosome can bind to the Shine-Dalgarno sequence and PyrC is expressed.
Further reading
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References
{{reflist
Cis-regulatory RNA elements