Microbial Genetics, Gene Expression, and Protein Quality Control

Microbial Genetics and Gene Expression

DNA Replication

DNA replication is a fundamental process in all living cells, ensuring the accurate transmission of genetic information. The process is semi-conservative, meaning each new DNA molecule contains one original strand and one newly synthesized strand.

• Anti-parallel Structure: DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Semi-conservative Replication: Each daughter DNA molecule consists of one parental and one new strand.

• DNA Polymerase Requirements: DNA polymerase requires an upstream 3' OH group to initiate synthesis; it cannot start de novo.

• RNA Primers: RNA polymerase can initiate synthesis without a primer, but DNA polymerase needs an RNA primer to provide the 3' OH.

• Replication Fork: At the origin of replication, two forks proceed bi-directionally.

• Leading vs. Lagging Strands: The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments (Okazaki fragments) due to the 5' to 3' directionality.

• Okazaki Fragments: Short DNA fragments on the lagging strand, initiated by RNA primers, later replaced by DNA and joined by ligase.

• End Replication Problem: Linear chromosomes (e.g., in eukaryotes) face issues at the ends, solved by telomeres; circular chromosomes (e.g., in bacteria) avoid this problem.

Example: Most bacteria have circular chromosomes, preventing loss of genetic material at chromosome ends during replication.

Central Dogma: Replication, Transcription, Translation

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Replication: DNA to DNA, catalyzed by DNA polymerase.

• Transcription: DNA to RNA, catalyzed by RNA polymerase.

• Translation: RNA to protein, catalyzed by ribosomes.

Example: Ribosomes translate mRNA into polypeptides using tRNAs.

Prokaryotic vs. Eukaryotic Gene Expression

Gene expression differs between prokaryotes and eukaryotes, mainly due to cellular compartmentalization.

• Prokaryotes: No nucleus; transcription and translation occur simultaneously. Ribosomes can bind mRNA as it is being transcribed.

• Eukaryotes: Transcription occurs in the nucleus; translation occurs in the cytoplasm or on the rough ER. mRNA must be processed and exported before translation.

• mRNA Structure: Prokaryotic mRNAs are often polycistronic (encode multiple proteins, usually related in function, organized in operons). Eukaryotic mRNAs are typically monocistronic (one gene per mRNA).

Example: The lac operon in Escherichia coli encodes several proteins involved in lactose metabolism on a single mRNA.

Translation: Mechanism and Regulation

Translation Initiation and Termination

Translation is the process of synthesizing proteins from mRNA templates. It involves initiation, elongation, and termination.

• Initiation: Ribosome binds to the ribosome binding site (Shine-Dalgarno sequence in prokaryotes) upstream of the start codon (AUG).

• Ribosome Structure: Composed of rRNAs (e.g., 16S rRNA) and proteins; the 16S rRNA is highly conserved and used for phylogenetic studies.

• Elongation: tRNAs bring amino acids to the ribosome, matching codons in the mRNA. The process involves the A (aminoacyl), P (peptidyl), and E (exit) sites.

• Termination: Occurs when a stop codon is encountered. Release factors (RF1, RF2) bind to the stop codon, catalyze release of the polypeptide, and RF3 facilitates ribosome recycling.

Example: Inhibition of RF1 or RF2 prevents polypeptide release; inhibition of RF3 prevents ribosome recycling but not polypeptide release.

Translation Errors and Quality Control

Errors in translation, such as missing stop codons, can result in nonfunctional proteins. Cells have mechanisms to address these issues.

• tRNA-mRNA (tmRNA): A hybrid molecule that rescues stalled ribosomes at the end of mRNAs lacking stop codons. It tags the aberrant protein for degradation and provides a stop codon for ribosome release.

• Protein Quality Control: Misfolded proteins are managed by chaperones (e.g., GroEL) or degraded by proteases.

• Chaperones: Facilitate correct folding by providing an environment that encourages proper hydrophobic and hydrophilic interactions.

• Proteases: Degrade irreparably misfolded proteins, recycling amino acids.

Example: GroEL chaperone uses ATP to refold proteins; if unsuccessful, the protein is sent to the proteasome for degradation.

Transcription: Initiation and Termination

Transcription Initiation

Transcription begins at specific DNA sequences called promoters, which determine where and in which direction RNA polymerase starts synthesizing RNA.

• Promoter Elements: In prokaryotes, the -35 and -10 elements are recognized by sigma factors, guiding RNA polymerase binding and directionality.

• TATA Box: A region rich in T and A bases, easier to unwind, often marks the transcription start site.

• Template vs. Non-template Strand: The mRNA sequence is complementary to the template strand and similar to the non-template (coding) strand.

Example: Sigma factor recognizes the -35 and -10 motifs, facilitating RNA polymerase binding and transcription initiation.

Transcription Termination

Transcription ends when RNA polymerase encounters specific signals. Two main mechanisms exist in prokaryotes:

• Rho-dependent Termination: Rho protein binds to GC-rich sequences in the RNA, moves toward RNA polymerase, and pulls the RNA away from the DNA, ending transcription.

• Rho-independent Termination: GC-rich sequences form hairpin loops in the RNA, followed by a poly-U (weak binding) region, which destabilizes the RNA-DNA hybrid and causes release.

Comparison Table:

Termination Type	Mechanism	Key Features
Rho-dependent	Rho protein binds RNA, moves to polymerase, pulls RNA off	Requires Rho protein, GC-rich pause site
Rho-independent	Hairpin loop forms, poly-U region destabilizes hybrid	GC-rich hairpin, poly-U region, no protein required

Example: Both mechanisms result in release of the RNA transcript, but differ in the means of separation.

Microbial Genetics: Gene Transfer

Vertical vs. Horizontal Gene Transfer

Microbes can acquire genetic material through vertical (parent to offspring) or horizontal (between unrelated cells) gene transfer.

• Vertical Gene Transfer: Transmission of genetic material from parent to daughter cells during cell division.

• Horizontal Gene Transfer: Acquisition of genetic material from other cells, not necessarily related or in the same generation.

Example: Horizontal gene transfer can introduce new traits, such as antibiotic resistance, into a population.

Mechanisms of Horizontal Gene Transfer

Three main mechanisms allow bacteria to exchange genetic material horizontally:

• Transformation: Uptake of free DNA from the environment by competent cells; DNA is incorporated into the genome.

• Conjugation: Direct transfer of DNA (usually plasmids) between cells via a sex pilus; requires F plasmid for fertility.

• Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).

Example: Streptococcus pneumoniae can acquire antibiotic resistance genes through transformation.

Additional info: The 16S rRNA gene is used as a molecular clock for bacterial taxonomy due to its slow mutation rate and conservation across species.