BIOL 438 lecture 20

What does transcription termination do

regulates transcription

what is attenuation

transcription termination in biosynthetic operons, first demonstrated in his and trp

what can Attenuation and Riboswitches do

they can Terminate Transcription Near the Start of Operons

what are “Intrinsic” Transcription Terminators

A stretch of self-complementary stem and loop structure followed by a run of 6 or more U’s in the mRNA

What happens when RNA polymerase encounters “Intrinsic” Transcription Terminators

it falls off the DNA and transcription is terminated.

What does the 2 3 loop do in the lac operon

it is the anitterminator loop, allowing for translation to keep going by RNA polymerase.

What does the 3 4 loop do in the lac operon

it is the transcription terminator loop, stopping translation by RNA polymerase.

How does attenuation control happen in the trp Operon Insufficient charged tRNATrp

due to the fact that translation and transcription happen at the same time in bacteria,

Ribosome stalls in the 2 trp operon and covers trp1. (translation happens sllowly due to not enough tRNATrp)

this leads to the 2 and 3 regions to bind, allowing for the anitterminator loop, allowing for translation to keep going by RNA polymerase.

How does attenuation control happen in the trp Operon with Adequate charged tRNATrp

due to the fact that translation and transcription happen at the same time in bacteria,

Ribosome reads the 1 trp operon and covers trp2. (translation happens quickly due to enough tRNATrp)

this leads to the 3 and 4 regions to bind, allowing for the terminator loop, stopping translation by RNA polymerase.

besides trp, what else is Attenuation also involved in

regulation of the

leu (leucine)

phe (phenylalanine)

thr (threonine) operons of E. coli.

PLT

Besides amino acid biosynthetic operons, what else is attenuation involved in?

uracil biosynthetic genes

how are riboswitches different from self-complementary stem and loop?

they are both transcription attenuations, but riboswitches do not need translation of the mRNA leader, but rather the Folding of mRNA 5’ leader sequence (the riboswitch)

in riboswitches, how is the folding pattern altered?

in response to mRNA binding of an effector (or regulatory) molecule

how can riboswitches regulate mRNA expression

Transcription termination

– Repression of translation initiation/elongation

– mRNA stability (degradation by RNases)

what can riboswitches bind to

metabolites, ions, and even tRNAs

riboswitch of Riboflavin (rib) Biosynthetic Operon of Bacillus subtilis

in low amounts of riboflavin, the mrna forms an antiterminator which allows RNA polymerase to continue transcription

in high amounts of riboflavin, the molecule FMN (made from riboflavin) binds to the riboswitch, which causes the terminator to form, causing RNA polymerase to stop transcription early

Regulation of Translation by an mRNA “Thermometer”

at low temp, mrna stem loops in the mrna to cover shine dalgarno sequence (which is needed to start translation)

at high temp, mrna stem undfolds, and the shine dalgarno sequence is available (which is needed to start translation)

Translation Regulation by Small RNA Molecules

Small (sRNAs) or noncoding (ncRNAs) RNAs

– do not function as mRNA, tRNA, or rRNA

– some (antisense RNAs) are complementary to mRNA and

function by base pairing

– may inhibit or enhance translation