Escherichia Virus T4: Structure, Life Cycle and FAQs

Table of Contents

• Introduction
• Classification
• Properties
• Genome
• Structure
• Process of Infection
• Reproduction and Life Cycle
• Future Applications
• Frequently Asked Questions

Introduction

Escherichia virus T4 is one of the seven E.coli phages (T1-T7) that belongs to the Myoviridae family. It has a double-stranded DNA, and it was considered as a model organism back in the 1940s, continuing even today. The ‘T-even’ phages, meaning T2, T4 and T6, are similar in structure, genetics and antigenic response.

Classification

Properties

Genome

• The double helical DNA genome is almost 169 kbp long and encodes 289 proteins.
• The genome is terminally redundant and possesses intron sequences like eukaryotes.
• The DNA undergoes replication by a rolling circle mechanism.
• In the Shine-Dalgarno sequence, GGAG is found abundantly in the early T4 genes.
• The DNA genome is packed in an icosahedral head that is known as a capsid.
• This virus can only undergo a lytic life cycle, not the lysogenic life cycle.

Structure

• The T4 bacteriophage belongs to the order Caudovirales because it possesses a contractile tail.
• A T4 bacteriophage is made up of a head, a tail and long tail fibres (LTFs) that are assembled independently before forming a mature phage.
• It is a large virus sizing approximately 90 nm wide and 200 nm long.
• The tail of the phage is hollow for easy transmission of the nucleic acid to the host cell.
• The head of a mature phage is 1150 Å long and 850 Å wide, which encapsulates the genome.
• The head is attached to a contractile head that is about 925 Å long and 240 Å in diameter.
• A hexagonal base plate that is 270 Å high and 520 Å in diameter is attached to the distal end of the contractile tail.
• The LTFs are 1450 Å long and attached to the periphery of the hexagonal baseplate. They are six in number.
• The LTFs are sensors that recognise the receptor molecules on the host.
• Additionally, there are six short tail fibres (STFs) that are attached beneath the baseplate. They unfold upon recognition with the host and increase the efficiency of the infection.

Process of Infection

The Escherichia virus T4 phage initiates infection by binding to the porin proteins and lipopolysaccharides on the surface of the E.coli cells with their long tail fibres. This initiates a recognition signal that is sent from the LTFs to the baseplate, and as a result, the short tail fibres unravel and attach themselves irreversibly to the cell surface of E.coli. Next, the baseplate changes confirmation and the tail sheath contracts to drill a puncture into the outer membrane of the bacteria.

Lysozyme enzymes from the tail are activated, and they destroy the peptidoglycan layer of the bacteria. The remaining membranes are also degraded, and then the viral genome is transferred from the head of the virus via the tail into E.coli cells.

Reproduction and Life Cycle

As stated above, the Escherichia virus T4 undergoes only a lytic life cycle. The lytic life cycle can be defined as the lysis of the host cell and release of the new viral progenies. As soon as the bacteriophages enter the host cell, they take over the machinery of the host cell and start to multiply rapidly.

The phage progeny multiplies to reach a certain number, after which the host cell bursts. The T4 phage has a burst size of almost 150 viral particles. The whole process of the infection and lysis of cells can be understood in five stages:

• Adsorption and penetration
• Arrest of the host gene expression
• Synthesis of enzymes
• DNA replication
• Formation of new viral particles.

The lytic life cycle takes place in a time span of 30 minutes at 37 ℃.

Future Applications

The complete genome of the T4 phage was sequenced in 2003. The amount of molecular information of the phage in hand could be used for medical advancements such as vaccine development, gene therapy, antibiotic agents and targeting the malfunctioning cells.

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Also Read:

• Structure and Life Cycle of Bacteriophage
• Difference between Lytic and Lysogenic Cycle
• Virus Life Cycle: Introduction, Life Cycle, FAQs