Bacterial Transcription and RNA

Transcription

DNA is converted to RNA

RNA sturcture

usually single stranded

RNA-RNA double strand called hairpins

contains ribose sugar

uracil replaces thymine

messenger RNA (mRNA)

encodes proteins

Ribosomal RNA (rRNA)

integral part of ribosomes

transfer RNA (tRNA)

shuttles amino acids

small RNA (sRNA)

regulates transcription or translation

tmRNA

frees ribosomes stuck on damaged mRNA

catalytic RNA

carries out enzymatic reactions

coding strand

usually on top and goes from 5’ → 3’

template strand

usually bottom strand and goes from 3’ to 5’

RNA is encoded from it

RNA polymerase

performs transcription of a strand of RNA from DNA

when does transcription begin?

when RNA pol binds to the promoter on DNA

promoter

a sequence of DNA upstream of the transcription start site (+)

what is RNA pol composed of in bacteria?

sigma factor and core polymerase

sigma factor

required for the initiation phase

helps the core enzyme detect the promoter, which signals the beginning of the gene

E. coli sigma factor

have sigma-70

recognizes consensus sequences at -10 and -35 positions, upstream of start of the RNA transcript (+1)

transcription three phases

initiation

elongation

termination

initiation

RNA pol binds to the promoter

this is followed by melting of the helix and synthesis of the first nucleotide of the RNA

elongation

the RNA is extended

termination

RNA pol detaches from the DNA, after the transcript is made

initiation of transcription

RNA pol, by the help of sigma factor, bind to the promotor region

results in unwinding of one helical turn

RNA polymerase then starts transcription

elongation of RNA transcripts

DNA unwind ahead forming a 17-bp transcription bubble

DNA unwinding result in positive supercoils ahead, which are removed by DNA topoisomerases

RNA pol move along the template, synthesizing RNA at ~45 bases/sec

Termination of transcription

all bacterial genes use one of two known transcription termination signals:

1. rho dependent

2. rho independent

Rho dependent termiantino signal

relies on a protein called Rho

contact between Rho and RNA pol cause termination

Rho independent terminatino signals

relies on inverted repeat sequence on DNA template

forms stem loop structure

operons regulatory region

includes both the promoter and operator

what happens if a repressor binds to the operator?

structural genes will NOT be transcribed

what happens if an activator binds to the regulatory region?

then genes will be transcribed

respressible operon

transcription is controlled by the binding of repressors to operator regions

tryptophan (trp)

example of a repressible operon

what happens when tryptophan accumulates in the cell?

tryptophan molecules bind to the trp repressor, which changes its shape, allowing it to bind to the trp operator

then RNA synthesis is blocked

inducible operon: lac operon

transcription is controlled by the presence of the substrate

what does the lac operon code for?

thee genes necessary for the hydrolysis of lactose into simple sugars glucose and galactose

what happens in the absence of lactose?

lac repressor is bound to the operator and physically prevents RNA polymerase from transcribing genes

what happens when lactose is present?

lactose binds to lac repressor and changes it shape so that it is no longer able to bind to the operator DNA

how eukaryotic transcription is regulated

an activator protein bound to an distant enhancer attracts RNA polymerase and the general transcription factors to the promoter

eukaryotic transcriptional activators also help initiate transcription by recruiting chromatin-modifying proteins

antibiotics must meet two fundamental criteria

they must kill or retard growth of a pathogen, and they must not harm the host

Rifamycin B (amycolatopsis mediterranei)

selectively binds to the bacterial RNA pol

inhibits transcription

Actinomycin D (actinomycete)

nonselectively binds to DNA

inhibits transcription