Ch. 36: RNA Synthesis & Regulation of Transcription in Bacteria

RNA

stores, transfers, and regulates genetic information (working copy of genome)

mRNA

encodes information from segments of DNA called genes to make proteins

rRNA

functional RNAs in ribosome, involved in translation

tRNA

carries amino acids for translation

RNA polymerase

core holoenzyme for transcription

sigma

promoter region, cofactor for RNA polymerase, finds promoter region, giving holoenzyme specificity

promoter

binding site for RNA polymerase

beginning of elongation

marked by release of sigma

template strand

read by RNA polymerase, complimentary sequence as RNA

coding strand

the gene, same sequence as RNA, what is transcribed (T’s instead of U’s)

transcription bubble

moves 50 nucleotides/second

intrinsic termination sequence

coded within RNA/DNA, simplest stop signal is GC rich sequence followed by stretch of U residues, doubles back and base pairs with itself to form hairpin structure

protein-dependent termination

requires rho protein, rho binds a stretch of 72 nucleotides on RNA, activated when G poor/C rich RNA sequences enter complex, breaks RNA-DNA hybrid once it reaches transcription bubble, stopping transcription

Ribonuclease III

cleaves spacer/non-coding region by excising 5S, 16S, and 23S rRNA precursors from the primary transcript by cleaving hairpin regions at specific sites (recognizes dsRNA)

to allow for greater structural and functional versatility

purpose of RNA modifications

tRNA processing

comes from nascent transcript, requires CCA terminal sequence for amino acid attachment, have unique bases that are enzymatically modified for use in translation

rRNA processing

some are methylated

constitutive expression

genes that are expressed all the time

regulated expression

genes are only expressed under certain circumstances

operon

a cluster of genes on the DNA that are all under the control of a single promoter

lac operon

allows E. coli to transcribe the genes needed to utilize lactose when glucose is scarce

without lactose present

lac-i (repressor) binds to the operator, preventing RNA polymerase from binding and transcribing mRNA

products of transcription of lac operon

B-galatosidase and allolactose

allolactose role

binds to lac-i repressor, reducing its affinity for the operator, allowing transcription of operon

low glucose, high lactose

we only want strong lac operon expression with what conditions?

Catabolite Activator Protein (CAP)

enhances transcription of lactose genes when bound to cAMP

relationship between glucose and cAMP concentrations

inverse