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Given the millions of different phages in the environment, phage genomes come in a variety of forms and sizes. RNA phages such as MS2 have the smallest genomes, with only a few kilobases. However, some DNA phages such as T4 may have large genomes with hundreds of genes; the size and shape of the capsid varies along with the size of the genome. [72]
Phages make up the majority of most viromes and are currently understood as being the most abundant organism. [5] Oftentimes scientists will look only at a phageome instead of a virome while conducting research. Variations due to many factors have also been explored such as diet, age, and geography.
Phages can also impact abiotic factors via the encoding of exotoxins (a subset of which are capable of solubilizing the biological tissues of living animals). Phage ecosystem ecologists are primarily concerned with the phage impact on the global carbon cycle , especially within the context of a phenomenon known as the microbial loop .
Superinfection exclusion, or protection against infection by multiple phages, can be conferred by prophage integration. [12] Additionally, phage-mediated recombination mechanisms may remodel the host chromosome and provide new ways for cells to regulate metabolism and gene expression, such as those involved in sporulation and competence.
Temperate phages (such as lambda phage) can reproduce using both the lytic and the lysogenic cycle. [4] How a phage decides which cycle to enter depends on a variety of factors. [5] For instance, if there are several other infecting phages (or if there is a high multiplicity), it is likely that the phage will use the lysogenic cycle.
Ff phages have been engineered for applications in biological and medical sciences. Many applications build on experiments [12] showing that the DNA sequence determining resistance to the antibiotic kanamycin can be inserted in a functional form into the non-coding intergenic sequence of fd phage DNA. Such modified phage are correspondingly ...
Phages fd, f1, M13 and other related phages are Ff phages, for F specific (they infect Escherichia coli carrying the F-episome) filamentous phages, using the concept of vernacular name. [45] Filamentous bacteriophage engineered to display immunogenic peptides are useful in immunology and wider biological applications.
The life cycle of lambda phages is controlled by cI and Cro proteins. The lambda phage will remain in the lysogenic state if cI proteins predominate, but will be transformed into the lytic cycle if cro proteins predominate. The cI dimer may bind to any of three operators, O R 1, O R 2, and O R 3, in the order O R 1 > O R 2 > O R 3.