ZOOHCC - 501: Molecular Biology (Theory)
Unit 3: Transcription and Regulatory RNAs
Unit 3: Transcription and Regulatory RNAs
RNA polymerase
RNA polymerase is an enzyme that copies a DNA sequence into an RNA sequence during the transcription process. RNA polymerases, complex molecules composed of protein subunits, direct the transcription process in which information stored in DNA molecules is copied into new messenger RNA molecules.
RNA polymerases are found in all species, but the number and composition of these proteins vary among taxa. For example, bacteria contain one type of RNA polymerase, while eukaryotes (multicellular organisms and yeast) contain three different types. Despite these differences, there are striking similarities between the transcriptional mechanisms. For example, all species require mechanisms that can regulate transcription to achieve spatial and temporal changes in gene expression.
Structure
Roger D. Kornberg (American biochemist) has provided a detailed molecular picture of RNAP enzymes at various stages of transcription. He won a Nobel Prize for this demonstration.
In prokaryotes, a single type of RNA polymerase enzyme consists of a core of five subunits. Two alpha subunits (36 kDa), one beta subunit (150 kDa), one beta prime subunit (155 kDa), and one small omega subunit. The core enzyme combines with the sigma factor to form the holoenzyme. Core enzymes form crab claws along the DNA being transcribed. RNA Pol enzymes contain metal cofactors such as zinc and magnesium that aid in the transcription process.
Eukaryotic RNAPs have a core structure similar to enzymes, but also some additional subunits.
Function
RNAP is a key enzyme that initiates transcription in both prokaryotes and eukaryotes. It induces specific DNA sequences on the DNA strand known as promoters to initiate transcription.
RNA Pol produces an RNA strand complementary to the template DNA strand, adding up to 2.4 million nucleotides to eukaryotes in a process called elongation.
RNA polymerase can generate the following RNA products:
mRNA (messenger RNA) that is translated into protein. RNA genes that produce RNA chains that do not code for proteins but are involved in other activities, or non-coding RNAs. They include:
A tRNA (transfer RNA) that adds an amino acid to a growing polypeptide chain during translation. rRNA (ribosomal RNA) is the component of the ribosome involved in translation. miRNAs (microRNAs) regulate gene activity. Ribozymes are enzymatically active RNA molecules.
Transcription Unit
A segment of DNA between the initiation and termination sites for transcription by RNA polymerase. Multiple genes may be contained in a transcription unit. A polycistronic message (q.v.) can be translated as is, and the translation product can later be enzymatically cleaved into two or more functional polypeptide chains. RNA is transcribed from the template strand of the gene in the 5' to 3' direction. However, when describing the nucleotide sequence of a particular gene, the convention has been adopted to give the same nucleotide sequence as the RNA transcript, except that each uridine is replaced by a thymidine. Elements to the left of the start site are called "from 5'" or "upstream" of the gene. Each element on the right is "three" or "downstream" from the gene. Nucleotides are numbered starting from the start site, with positive values on the right and negative values on the left. For genes, the binding site for RNA polymerase II is from nucleotides -80 to -5 and the first intron may contain nucleotides +154 to +688. Chronology, 1967, Taylor et al., Coding Strand, Miller trees, polyprotein, RNA polymerase, strand terms.
