Microbiology Chapter 8

Front 1

- Sigma factor

Back 1

A specificity factor that directs RNA polymerase to bind to the promoter

Front 2

Initiation (Fig. 8.6)

Back 2

- The core RNA pol constantly binds and releases DNA at random
- It will not remain tightly bound unless the sigma factor is attached
- The binding of the holoenzyme to the promoter forms the closed complex
- The RNA pol then unwinds the DNA to form the open complex
- RNA pol then inserts the first ribonucleotide triphosphate, which is usually a purine
- After the transcription of about 9 bases, the sigma factor is released

Front 3

Elongation

Back 3

- The core RNA pol continues along the DNA
- As the DNA unwinds and transcription occurs, a 17 bp transcription bubble advances
along the template
- Positive supercoils are generated ahead of the advancing bubble
- These are alleviated by DNA gyrase

Front 4

Termination (Fig. 8.8)
1) Rho-dependent termination

Back 4

- Relies on an ill-defined sequence at the 3’ end of the gene that is a strong pause site
- The Rho protein binds to a GC-rich region as a hexamer
- RNA wraps around Rho, and so Rho is pulled towards the paused RNA pol
- Once Rho touches the RNA pol, an intrinsic RNA-DNA helicase activity
built into Rho releases the completed transcript

Front 5

Termination (Fig. 8.82)
Rho-independent termination

Back 5

- Relies on a GC-rich sequence that is capable of intra-strand basepairing
- It creates a STEM loop structure followed by a poly-U sequence
- The hairpin causes RNA pol to pause
- At this point, the RNA is bound to the DNA only by the U-A bonds
- These are very weak bonds
- And so, the RNA-DNA hybrid melts spontaneously halting transcription

Front 6

1) Transfer RNA

Back 6

- Small in size
- Long-lived, with half-life measured in hours
- The secondary structure is a clover-leaf model (Fig. 8.10)
- Processed after transcription
- Is not translated
- Carries amino acids
- Contains unusual bases (Fig. 8.12)

Front 7

2) Ribosomal RNA

Back 7

- Three types
- Long-lived
- The secondary structure is highly complex (Fig. 8.17)
- Processed after transcription
- Is not translated
- Combine with specific proteins to form
- Contains unusual bases (Fig. 8.12)

Front 8

3) Messenger RNA

Back 8

- Variable in size
- Contain base triplets that constitute genetic code
- Short-lived, with half-life measured in minutes
- Polycistronic = Encodes information for more than one gene
- Contains only coding regions
- Not processed before translation

Front 9

4) Small RNAs

Back 9

- Are not translated
- Variable half-life
- Regulate gene expression

Front 10

5) tmRNA

Back 10

Frees ribosomes stuck on damaged mRNA

Front 11

6) Catalytic RNA

Back 11

- Usually associated with a protein cofactor
- Carries out enzymatic reactions
- E.g.: RnaseP, which is involved in processing of tRNA transcripts

Front 12

• The main players in protein synthesis are:

Back 12

- mRNA
- tRNA
- ribosomes