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In this article we will discuss about the structure of bacteriophage.
Bacteriophage ØX174 belongs to the genus Micro virus of the family Microviridae from the Group II (ssDNA Viruses).
The bacteriophage ØX174 was discovered by R.L. Sinsheimer at California Institute of Technology. It is one of the ssDNA phages of E.coli which has been most extensively studied. The phage particles are naked and icosahedral having a diameter (without spike) of 24-29 nm. The weight of a virus particles is 6.2 X 106 Daltons.
The capsid is made up of capsomers, each consisting of five structural units. Morphologically, the capsomers are probably angular, hollow and pentagonal from the centre of which projects a single spike situated at one apex of the icosahedron (Fig 18.8). Hence, there are 12 spikes in one phage particle.
Individual spike is constituted by H protein (encoded by one gene) and G protein (encoded by five genes). These interact with F gene protein. The H gene protein assists the adsorption of phage to the bacterial cell, in addition to functioning as pilot protein and helping the injection of DNA into the bacterial cell.
In 1962 Walter Friers had already demonstrated the physical and covalently closed circularity of ØX174 DNA. The capsid encloses a single and circular (+) ssDNA (molecular weight 1.7 x 106 Daltons) consisting of eleven genes arranged in an order A, B, C, D, E, J, F, G, H, A* and K. Gene A* and K overlap to each other. The gene B lies within gene A, and gene E is present within gene D.
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The gene C overlaps with genes A and D. Therefore, from two genes four proteins are encoded instead of two. Sanger (1977) have shown that the phage contains .5386 nucleotides and mapped the nucleotide sequence of ØX174 (Fig. 18.9).
They discussed the function of these genes as below:
Gene A : Replication of phage by introducing single stranded break in the DNA.
Genes B, C, D: Synthesis and packaging of ssDNA of progenies.
Gene E : Bacterial cell lysis.
Gene J : Internal protein.
Gene F : Major capsid protein
Gene G : Synthesis of spike protein
Gene H : Synthesis of spike protein, adsorption, injection of phage DNA.
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Gene A* : Overlaps
Gene K : Overlaps
This phage consists of a very small amount of single stranded circular DNA having 11 genes in 5386 bases. The GC content is 44% and 95% of nucleotides are coding genes. Several genes express similar function in two groups.
All genes are transcribed clockwise. Enumeration of phage DNA begins with the unique Pstl site and continues clockwise around the viral (+) strand in the 5′ → 3′ direction (Fig. 18.9). The map shows enzymes that cut ØX174 DNA once. The coordinates refer to the position of the first nucleotide in each recognition sequence.
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The DNA strand packaged into the virion is termed the ‘plus’ strand. After entering the cell, ØX174 DNA is used as a template for minus-strand synthesis, producing double-stranded DNA. The conversion of plus DNA strands to double-strands does not require any of the phage genes to function. The double-stranded DNA can then be transcribed, resulting in synthesis of phage-encoded proteins.
