Microbiology Chapter #10 - Transcription, Translation, and Translocation

Bacterial Protein Coding Gene

- Polynucleotide sequence in the DNA

- Promoter region tells RNA polymerase where to start

- Reading Frame are genes that need to be replicated

Transcription: Initiation

- RNA polymerase and Sigma Factor bind to promoter region

- DNA unwinds and denatures "transcription bubble"

- RNA polymerase begins to synthesize complementary nucleotides 5 to 3

- Sigma Factor dissociates

Transcription: Elongation

- RNA polymerase moves along, adding nucleotides

- Previous stretch of DNA reannealed

Transcription: Termination

- RNA polymerase recognizes signal and stops, releasing the pre-mRNA transcript

Bacterial Promoters

- show RNA polymerase where to bind and begin

Sigma Factors

- Recognize and bind to promoter sequence, signaling RNA polymerase to bind as well

Factor-Dependent Termination

- The Rho Factor binds to a site on mRNA called "rut" and follows RNA polymerase along strand

- RNA polymerase pauses at rho-dependent pause site, and rho catches up and forces it off of the DNA

Factor-Independent Termination

- RNA polymerase transcribes inverted repeat but slows down at the A-rich sequence

- The inverted repeated folds back on itself in a hairpin strucutre, and RNA polymerase falls off

Universal Genetic Code

- Every living organism uses the same system to code for protein

Wobble

- Reduces the amount of different tRNAs needed for translation

- Only the first two basepairs must be correct

tRNA

- Contain anticodons complementary to codons on mRNA

- Acceptor end where amino acid can attach

- Hairpin stucture

Translation Initiation Complex

- ATP group is added to an amino acid, then added to tRNA

Bacterial Ribosomes

- Large 50S, Small 30S

- 70S Ribosome

- 3 sites for translation: A, P, and E

Initiator tRNA

- Carries N-formyl methionine

- Through GTP, large subunit of the ribosome is attached and the first tRNA attaches to the P site

A, P, and E sites

- tRNA with the next amino acid enters the A site

- Peptide bond forms between the amino acids of the tRNAs in A and P site, creating chain

- Empty tRNA moves to E site and other tRNA moves to P

Molecular Chaperones

- Help proteins fold from secondary to tertiary

- Trigger Factor

Translocation

- Proteins are moved across the cytoplasm or to the plasma membrane

Secretion

- Proteins are moved from the cytoplasm to the external environment

Translocation Systems

- Transports unfolded proteins across the membrane or into the membrane

- Sorted and targeted by signal peptide

- Signal Recognition Particle

Secretion (Sec) System

- Unfolded proteins to cytoplasmic membrane or periplasmic space

- SecA protein recognizes less hydrophobic signal peptides

- SecY, SecE, SecG form a channel in the membrane

- SecA threads peptide through channel, acts as a motor

- SecB keeps the protein unfolded

- SecDF uses the proton motive force to help

Twin Arginine Translocase (Tat) System

- Two consecutive arginines in a signal peptide

- Secretes only folded proteins

- Protein Docking Complex recognizes signal and escorts protein to pore complex with PMF

Maturation Steps

- Signal peptide removed by signal peptidase

Secretion of Protein in Gram Negatives: 2-step Processes

- Must pass through cytoplasmic membrane, periplasmic space, and outer membrane

- Step 1: Tat or Sec system

- Step 2: Type II (Proteobacteria anchored in both membranes, joined by pseudopilus), V (barrel structure in outer membrane), and IX (bacteriodetes)

Secretion of Protein in Gram Negatives: 1-step Processes

- Type I: ABC membrane fusion, barrel structure

- Type III: Injectosomes

- Type IV: DNA transfer

- Type VI: contractile weapons

- Type VII: Mycobacterium