5e Chapter28 2003

5e Chapter28 2003 - 2002, 2003 by Changwon Kang Chapter 28:...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
© 2002, 2003 by Changwon Kang Chapter 28 1 Chapter 28: RNA Synthesis and Splicing A. Transcription: DNA content is moved (tran-) to and rewritten (-scribed) in RNA (1) The basic biochemistry is common to prokaryotes and eukaryotes. (2) It takes place in three stages: initiation, elongation, and termination. (3) DNA-dependent RNA polymerases perform multiple functions. They search DNA for promoters (a cis-acting element) to start RNA chain de novo. They unwind a short stretch of duplex to produce a single stranded template. They select the correct rNTP and catalyze the formation of a phosphodiester bond. This process is repeated for elongation of RNA. RNA polymerase is completely processive. They detect terminator sequences (a cis-acting element). They interact with transcription factors (trans-acting elements) that modulate the rate. (4) RNA is elongated by the attack of its 3'-OH at the α -phosphate of incoming NTP and the subsequent hydrolysis of pp i . It is thermodynamically favorable. (5) The chemistry is the same for all RNAs. Their processes differ mainly in regulation, posttranscriptional processing, and specific RNA polymerase . B. Prokaryotic Transcription (E. coli) (1) RNA polymerase holoenzyme = core enzyme ( α 2 ββ ') + σ subunit. The catalytic site in core enzyme has two metal ions (one bound to the enzyme, and the other coming in with rNTP and leaving with pp i ). Three Asp residues hold the ions. The σ subunit is involved in initiation but dissociated during transition to elongation stage. (2) Initiation The σ subunit of holoenzyme binds specifically to promoter sequences through H-bonds. (The σ alone does not bind.) The search is very fast, because the enzyme walks (slides) on DNA in one dimension rather than in three-dimensional dissociation/association. Promoters have consensus sequences. E. coli has several σ factors, each of which binds to its own type of promoter sequence. σ 70 σ 32 σ 54 Promoters differ markedly in their efficacy, transcription frequency, or strength: every 2 seconds to 10 minutes. Strong promoters have sequences that are similar to the consensus, and weak promoters have multiple substitutions. The distance between conserved sequences is important. Promoter sequences can be identified by various ways such as footprinting, mutagenesis, derivation of consensus sequences, etc. Each bound polymerase unwinds 17 bp (1.6 turns of B-DNA). This transition (isomerization) from the closed complex to the open complex is essential. Most, but not all, promoters are stimulated by negative supercoiling. RNA synthesis starts de novo without need for a primer. RNA is synthesized in 5' 3' direction. The first, 5'-end nucleotide of synthesized RNA has ppp. About 10 nt long RNA is synthesized from the initially formed bubble. This initial synthesis is
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 4

5e Chapter28 2003 - 2002, 2003 by Changwon Kang Chapter 28:...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online