Question

Transcriptional Polarity (thought problem)

Happens in operons à transcription of “later” genes depends on the translation of the ones before them

–A nonsense mutation (“early stop”) in the first gene can prevent not just that product, but all of the other gene products from being made

•This effect requires that transcription and translation be coupled

•Genes on a polycistronic mRNA all have their own translation start and stop sites though. If the RNA encoding those other genes is present, they should still get translated

Because of that early mutation, the rest of the mRNA is not produced

•However, this effect is sometimes Rho-dependent: mutations in Rho can suppress polarity

–The “early stop” causes transcription to terminate early, at one of the internal terminators. If Rho is normally required for termination here, termination won’t happen if Rho is non-functional

•Makes sense, but, if these genes all have their own internal terminators, why don’t they cause early termination all the time?

–What else is happening?

Answer 1

• Transcription termination may be Rho dependent or Rho independent.
• In Rho dependent termination: Rho protein binds to the terminator sequence without secondary structure, hair- pin structures further formed to halt RNA polymerase.

• Rho protein belongs to RAS super family of GTPases.
• Rho GDI regulates the activity of Rho GTPase, which in turn regulates the rapid assembly of actin (F-filaments) necessary for wound healing.
• GDI is Guanine nucleotide (GDP) Dissociation inhibitor.
• GDI when bound to GTPase retain them in inactive form.
• Guanine nucleotide exchange factor (GEF) promote the exchange of GDP for GTP by the GTPases, which activates them.
• RHO-GTPases can act on several targets.
• Two major functions include: assembly of actin-myosin rings and RHO associated kinases.
• Mutation of RHO protein genes will prevent RHO-GTPase signalling and affect functions like wound healing, adhesion, cell polarity, membrane trafficking, cytokinesis.