DOI link for B6 Bacteriophages
B6 Bacteriophages book
Bacteriophages Bacteriophages (phages) are viruses that infect bacteria. Many different types of bacteriophage exist which tend to be species speciﬁ c infecting only certain bacterial hosts. Like other viruses, their structure consists of an inner nucleic acid genome surrounded by an outer protective protein coat. Three types of phage structure are seen (Figure 1): (i) icosahedral, in which the individual polypeptide molecules form a geometrical structure that surrounds the nucleic acid – an example is the phage MS2 that infects E. coli; (ii) ﬁ lamentous or helical, in which the polypeptide units are arranged as a helix to form a rodlike structure – an example is the E. coli phage M13; (iii) head and tail, the most complex phage structure consisting of an icosahedral head and a ﬁ lamentous tail that allows the phage nucleic acid to enter the cell – examples of this type are the E. coli phages T4 and λ. The nucleic acid in phages may be DNA or RNA which may be single-stranded or doublestranded. Usually all the genes are present on a single nucleic acid molecule; however, in some RNA phages the genes are present on more than one molecule and these phages are said to be segmented. Phage genomes vary greatly in size from just a few kilobase pairs (kbp) to about 150 kbp. The number of genes varies roughly in proportion to genome size. The genome of the phage M13 is about 6 kbp and contains just 10 genes. Larger phages, especially those with complex structures (e.g. head and tail), have many more. For example, phage T4 at 166 kbp has 150 genes. Phage genes encode proteins required
proteins. Lysozyme is produced at the end of the lytic cycle. Simple phages such as φX174 express all their genes at the same time. Other phages have early and late phases of expression. Early gene expression is associated with replication of the phage genome and late gene expression with the synthesis of structural proteins. Expression of early genes activates the expression of later genes by modifying the speciﬁ city of the host RNA polymerase.