Bacterial Genome Replication and Expression: Study Notes for Genetics Students

Bacterial DNA Replication

Overview of DNA Replication

DNA replication is a fundamental process in bacteria, ensuring the accurate transmission of genetic information to daughter cells. The process is semi-conservative, meaning each daughter cell receives one old and one new DNA strand.

• Proteins ensure accuracy during replication.

• Each DNA strand serves as a template for synthesis of a complementary strand.

• Replication is semi-conservative.

Patterns of DNA Synthesis

Bacterial DNA is typically circular and undergoes bidirectional replication from a single origin.

• Replication fork: Site where DNA is unwound.

• Replicon: Portion of DNA replicated as a unit (entire genome in bacteria).

Replication Machinery in E. coli

The replication machinery consists of multiple proteins, each with a specific function in the replication process.

• DNA polymerase: Catalyzes synthesis of complementary DNA strand in the 5’ to 3’ direction, forming phosphodiester bonds.

• Requires a template, primer, and dNTPs (deoxynucleotide triphosphates).

• E. coli has five DNA polymerases, with polymerase III playing the major role.

• DNA polymerase III holoenzyme is a complex of 10 proteins, with three core enzymes for synthesis and proofreading.

Protein	Function
DnaA	Initiation of replication; binds origin of replication (oriC)
DnaB	Helicase; unwinds DNA strands
DNA gyrase	Relieves supercoiling; introduces negative supercoiling
SSB proteins	Bind single-stranded DNA; keep strands apart
DnaC	Helicase loader; helps DnaB bind to DNA template
DNA primase	Synthesizes RNA primer
DNA polymerase III holoenzyme	Main DNA synthesis and proofreading
DNA polymerase I	Removes RNA primers; fills gaps
Ribonuclease H	Removes RNA primers
DNA ligase	Links DNA fragments
Topoisomerase IV	Separates chromosomes after replication

Events at the Replication Fork

Replication fork dynamics involve several proteins and steps:

• DnaA binds oriC, causing strand separation.

• DnaB and helicases separate strands; SSBs attach.

• Primase synthesizes RNA primer.

• Lagging and leading strands are synthesized.

• DNA polymerase I removes RNA primers and fills gaps.

• DNA ligase joins Okazaki fragments.

Okazaki Fragments and DNA Ligase

The lagging strand is synthesized in short fragments called Okazaki fragments, each requiring a new primer. DNA ligase forms a phosphodiester bond between fragments.

Proofreading

DNA polymerase III carries out proofreading by removing mismatched bases from the 3’ end of the growing strand via exonuclease activity. This ensures high fidelity but is not 100% efficient.

Termination of Replication

Replication stops when the replisome reaches the termination site (ter). Catenanes may form if daughter chromosomes do not separate, requiring topoisomerases and recombinases.

Gene Structure and Expression

Gene Structure

A gene is the basic unit of genetic information, consisting of a nucleic acid sequence coding for a polypeptide, tRNA, or rRNA. Genes have a fixed start and end point, and codons in mRNA code for single amino acids.

Protein-Coding Genes

The template strand of DNA directs RNA synthesis, read in the 3’ to 5’ direction. The coding strand has the same sequence as mRNA (except thymine).

Promoter and Regulatory Sequences

The promoter is located at the start of the gene and serves as the recognition/binding site for RNA polymerase. Regulatory sequences control gene expression and may be located upstream or downstream of the gene.

Transcription

Transcription in Bacteria

Transcription is the synthesis of RNA under the direction of DNA. Three types of RNA are produced: mRNA, tRNA, and rRNA. Polycistronic mRNA is common in bacteria, containing directions for multiple polypeptides.

RNA Polymerase Structure

Bacterial RNA polymerase consists of a core enzyme (β', β, α, α, ω) and a sigma (σ) factor. The holoenzyme (core + sigma) initiates transcription.

Transcription Initiation

Initiation occurs at the promoter, which contains specific sequences (e.g., Pribnow box) recognized by RNA polymerase. Only a short segment of DNA is transcribed.

Transcription Elongation

RNA polymerase unwinds DNA, forming a transcription bubble. A temporary RNA:DNA hybrid is formed, and the sigma factor dissociates.

Transcription Termination

Termination occurs when RNA polymerase dissociates from DNA. Some terminators require the rho factor, which has helicase activity.

Translation and the Genetic Code

The Genetic Code

The genetic code consists of codons, each specifying an amino acid. The code is degenerate, with multiple codons for some amino acids. The start codon is always AUG, and stop codons are UAA, UGA, and UAG.

Amino Acid	Abbreviation (3 Letters)	Abbreviation (1 Letter)
Alanine	Ala	A
Arginine	Arg	R
Asparagine	Asn	N
Aspartic Acid	Asp	D
Cysteine	Cys	C
Glutamic Acid	Glu	E
Glutamine	Gln	Q
Glycine	Gly	G
Histidine	His	H
Isoleucine	Ile	I
Leucine	Leu	L
Lysine	Lys	K
Methionine	Met	M
Phenylalanine	Phe	F
Proline	Pro	P
Serine	Ser	S
Threonine	Thr	T
Tryptophan	Trp	W
Tyrosine	Tyr	Y
Valine	Val	V

Exceptions to the Genetic Code

Some organisms use alternative codons or incorporate rare amino acids such as selenocysteine and pyrrolysine.

Codon	Amino Acid Inserted	Where Observed
AGA and AGG	Stop	Mammalian mitochondria
AGA and AGG	Serine	Invertebrate mitochondria
AUA	Methionine	Mammalian, invertebrate, and yeast mitochondria
CUA	Threonine	Yeast mitochondria
CUG	Serine	Some fungi
UAA and UAG	Glutamine	Some protists
UAG	Pyrrolysine	Some methanogens and bacteria
UGA	Selenocysteine	Members of all three domains
UGA	Tryptophan	Mammalian, invertebrate, and yeast mitochondria
UGA	Glutamine	Mycoplasma bacteria

Translation

Translation Process

Translation is the synthesis of polypeptides directed by the sequence of nucleotides in mRNA. It occurs at the ribosome and involves tRNA and rRNA.

• Direction: N-terminal to C-terminal

• Polyribosome: Complex of mRNA with several ribosomes

tRNA Structure and Function

tRNA has a tertiary structure due to base pairing, with an anticodon complementary to the mRNA codon and a 3’ end that binds the amino acid.

Amino Acid Activation

Amino acids are attached to tRNA by aminoacyl-tRNA synthetases, each specific for a single amino acid and its cognate tRNAs.

Ribosome Structure

Bacterial ribosomes are 70S, composed of 30S and 50S subunits. The translational domain is responsible for translation.

tRNA Binding Sites

• A site: Binds incoming aminoacyl-tRNA

• P site: Binds initiator tRNA or tRNA attached to growing polypeptide

• E site: Briefly binds empty tRNA before it leaves

Role of rRNA in Translation

• 16S rRNA: Binds Shine-Dalgarno site for initiation

• 23S rRNA: Catalyzes peptide bond formation (peptidyl transferase)

Initiation of Protein Synthesis

Initiation involves ribosome subunits, initiator tRNA (N-formylmethionine in bacteria), and initiation factors. The Shine-Dalgarno sequence aligns with 16S rRNA.

Elongation and Termination

• Elongation: Sequential addition of amino acids via aminoacyl-tRNA binding, transpeptidation, and translocation.

• Termination: Occurs at stop codons (UAA, UAG, UGA) with the aid of release factors and GTP hydrolysis.

Protein Maturation and Secretion

Protein Folding and Chaperones

Protein function depends on proper folding, often assisted by molecular chaperones (e.g., heat-shock proteins).

Protein Splicing

Inteins are removed from polypeptides before folding; exteins remain in the protein.

Protein Translocation and Secretion

Proteins are translocated across membranes via Sec and Tat systems. Secretion systems (Type I-VI) are used for exporting proteins, including virulence factors.

Sec-Dependent Pathway

• Major pathway for transporting proteins across plasma membrane

• Secreted proteins synthesized as pre-proteins with signal peptide

• Chaperone proteins (SecB) keep pre-proteins unfolded

• secY, secE, secG form membrane channel; secA translocates pre-protein

Tat System

• Twin arginine translocase; moves folded proteins with twin arginine residues

• Works with Type II secretion system

Other Secretion Pathways

• Type I: Related to ABC transporters; secretes toxins, proteases

• Type IV: Secretes proteins and DNA during conjugation

• Type II, III, V, VI: Unique to Gram-negative bacteria; transport virulence factors

Summary Table: Key Steps in Bacterial Genome Replication and Expression

Process	Key Proteins/Enzymes	Function
DNA Replication	DNA polymerase III, DnaA, DnaB, SSB, DNA gyrase, primase, ligase	Synthesis and proofreading of DNA
Transcription	RNA polymerase (core + sigma), promoter, terminator	Synthesis of RNA from DNA template
Translation	Ribosome, tRNA, rRNA, aminoacyl-tRNA synthetase	Synthesis of polypeptide from mRNA
Protein Secretion	Sec, Tat, Type I-VI systems	Translocation and secretion of proteins

Example: E. coli uses DNA polymerase III for replication, RNA polymerase holoenzyme for transcription, and ribosomes for translation. Sec and Tat systems are used for protein secretion.

Additional info: Academic context was added to clarify the roles of enzymes, the structure of genes, and the mechanisms of transcription and translation, as well as the significance of protein secretion pathways in bacteria.