Structure and Organization of Bacterial Genes and Genomes

The Structure of Bacterial Genes and Genomes

Introduction

Bacterial genomes are highly organized and compact, consisting of chromosomes and often plasmids. Their structure and gene organization differ significantly from those of eukaryotes and viruses, impacting replication, gene expression, and adaptation.

Distinct Aspects of Animal Viruses vs. Bacteria

Viral Genome Localization and Infection Types

• Genome Localization: Animal virus genomes can localize to specific organelles within the host cell, such as the nucleus or endosomes.

• Entry Mechanisms: Viral genomes may enter the cell via endocytosis, fusion with endosomes, or direct transport into the nucleus.

• Types of Infection: Animal viruses can cause acute (lysis), chronic (persistent), latent (delayed), or transformative infections (leading to cancer).

Additional info: Unlike bacteria, animal viruses rely on host cell machinery for replication and may integrate into host genomes.

Bacterial Genome Composition

Chromosomes and Plasmids

• Chromosome: Most bacteria have a single, circular chromosome, though some species possess multiple or linear chromosomes.

• Plasmids: Plasmids are extrachromosomal DNA elements that replicate independently. Most are circular, but some are linear. Bacteria may have no plasmids, one plasmid, or multiple plasmids.

Additional info: Plasmids often carry genes that confer selective advantages, such as antibiotic resistance.

Genome Structure Variation Among Bacteria and Archaea

Comparative Genomic Organization

The structure and size of bacterial and archaeal genomes vary widely among species. The following table summarizes key examples:

Organism (Strain)	Genomic Chromosome(s) (kb)	Plasmid(s) (kb)
Mycobacterium tuberculosis	4,400	-
Mycoplasma genitalium	580	-
Burkholderia cepacia	3,870 + 3,217 + 876	93
Escherichia coli K-12	4,600	-
Synechocystis PCC 7120	6,370	110 + 190 + 410
Borrelia burgdorferi	911	21 plasmids (9–58 kb)
Agrobacterium tumefaciens	2,840 + 2,070	214 + 542

Additional info: Archaeal genomes also show diversity, with some species possessing multiple chromosomes and plasmids.

Prokaryotic Genome Size and Gene Content

Genome Size Range

• Example: Escherichia coli strain MG1655 has a genome of 4,639,675 base pairs (bp), equivalent to 4,640 kilobase pairs (kb) or 4.6 megabase pairs (Mb).

• Gene Count: Contains 4,288 open-reading frames (ORFs), each typically ~1 kb (1,000 bp) in length.

• Range: Prokaryotic genomes can be as small as 0.1 Mb (e.g., Nasuia deltocephalinicola) or as large as 14.8 Mb (e.g., Sorangium cellulosum).

Additional info: The human genome is much larger (~6,300 Mb), with the largest known genome being ~149,300 Mb.

Structure of Circular Prokaryotic Chromosomes

Supercoiling and DNA-Binding Proteins

• Supercoiling: Prokaryotic chromosomes are highly compacted by negative supercoiling, facilitated by enzymes such as DNA gyrase (a type of topoisomerase).

• DNA-Binding Proteins: Histone-like proteins anchor and organize DNA into domains, allowing independent coiling and efficient packaging.

• Functional Impact: Negative supercoiling favors DNA unwinding, promoting the initiation of gene expression.

Additional info: E. coli chromosomes are organized into over 100 domains.

Gene Organization and Expression in Prokaryotes

Monocistronic vs. Polycistronic Genes

• Gene: A region of DNA encoding a protein or functional RNA.

• Monocistronic RNA: Transcribed from a single gene, encodes one polypeptide or functional RNA.

• Polycistronic RNA: Transcribed from an operon, encodes multiple proteins under the control of a single promoter.

• Operon: A cluster of genes with related functions, expressed together for coordinated regulation (e.g., biosynthetic pathways).

Additional info: About 30% of E. coli genes are organized in operons, which are space-efficient and facilitate horizontal gene transfer.

Prokaryotic vs. Eukaryotic Chromosomes

Key Differences

• DNA Usage: Prokaryotes have little repetitive DNA and minimal non-coding DNA compared to eukaryotes (>90% non-coding in eukaryotes).

• Introns: Bacteria lack introns; Archaea have few introns.

• Pseudogenes: Few pseudogenes in prokaryotes.

• Horizontal Gene Transfer: Extensively used by prokaryotes for genetic diversity.

Plasmids: Structure, Replication, and Function

Properties and Replication

• Definition: Plasmids are genetic elements that replicate independently of the chromosome.

• Size: Usually small (1,000 to 100,000 bp), but can vary.

• Distribution: Found in Bacteria, Archaea, and some eukaryotes (yeasts, filamentous fungi).

• Replication Mechanisms: May replicate by rolling circle or other mechanisms; can be transferred to new cells via conjugation.

Functional Roles

• Non-Essential: Plasmids are not required for basic cellular functions but often confer survival advantages.

• Examples:

  • Antibiotic resistance (e.g., R plasmids)

  • Virulence factors

  • Biodegradation of compounds

• Horizontal Transfer: Plasmids can move between cells, spreading beneficial traits.

Antibiotic Resistance and R Plasmids

Mechanisms and Impact

• Resistance Genes: Encode enzymes that degrade antibiotics or detoxify heavy metals.

• R Plasmids: Plasmids with multiple resistance genes (e.g., R100 plasmid confers resistance to mercury, sulfonamides, streptomycin, tetracycline, and chloramphenicol).

• Rapid Spread: Pathogens with R plasmids can quickly disseminate resistance throughout a population, even to antibiotics not previously used.

Organism of the Day: Bacillus thuringiensis

Key Features

• Gram-positive, spore-forming bacterium

• Contains plasmids encoding insecticidal proteins (toxic to insects)

• Insecticidal proteins are produced during sporulation and form a parasporal crystal

• Genes for these toxins have been engineered into plants for pest resistance (e.g., European corn borer)