Moonlighting Outside of the Box
Bacteriophage Moonlighting Proteins in the Control of Bacterial Pathogenicity
Janine Z. Bowring 1, Alberto Marina2,3, Jose R. Penades 1, and Nuria Quiles-Puchalt1
- 11nstitute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, UK
- 2 Instituto de Biomedicina de Valencia, IBV-CSIC, Valencia, Spain
- 3 CIBER de Enfermedades Raras (CIBERER-ISCIII)
Bacteriophages (phages) are viruses that infect and replicate within bacterium following injection of their genome into the bacterial cell. Phages are basically composed of a proteic particle that encapsulates the DNA or RNA genome. They are found with enormous diversity in genome composition, virion structures, and lifestyles, being widely distributed in a great variety of habitats. With a global estimated population of 1030, phages are considered the most abundant organisms in the biosphere (Brussow and Hendrix 2002).
Phages play critical roles in bacterial virulence, ecology and diversity. Thus, bacteriophages can alter the genome architecture of bacteria as they serve as points for genomic rearrangement due to their mosaic nature (Brussow et al. 2004). Integrated temperate phages can influence the global regulators of the host, allowing them to adapt to a specific niche (Clokie et al. 2011), they confer protection to possible lytic infections by other phages (Berngruber et al. 2010), and can be involved in different processes of ecological competition between different bacterial species racing for the same niche (Selva et al. 2009). Bacteriophages can also contribute greatly to the pathogenicity of bacteria as they encode for different virulence factors and for many of the toxins that cause specific toxin- mediated diseases (toxinoses). The best known of these toxins are responsible for diseases such as diphtheria, cholera, dysentery, botulism, food poisoning, scalded skin syndrome, necrotizing pneumonia, or scarlet fever (Boyd et al. 2012). As well as their own transfer, phages can mobilize host genomic segments and other mobile genetic elements (MGEs) by classical generalized transduction. This widespread mechanism, common to most bacteria, allows the transfer of any
Moonlighting Proteins: Novel Virulence Factors in Bacterial Infections, First Edition. Edited by Brian Henderson.
© 2017 John Wiley & Sons, Inc. Published 2017 by John Wiley & Sons, Inc.
gene from one bacterium to another and greatly contributes to bacterial evolution (Penades et al. 2015).
Despite the wide diversity in size and genomic content of bacteriophages, a common characteristic to all of them is that they encode the essential genes needed to complete their life cycle and all the factors required to successfully take advantage of the host cellular machineries. The constraints imposed by the limited amount of genomic content that the phage capsids can package renders the question of which genes bacteriophages should encode a matter of vital importance. It is therefore assumed that the genes encoded by a bacteriophage should have a vital function for the phage so that the genetic content is optimized to obtain the best outcome. This restriction in genome size could hinder the acquisition of new and adaptive functions, so the acquisition of a novel function by an already encoded protein could be highly beneficial for the bacteriophage. On that account, it could be expected that within the different bacteriophage genomes studied so far, plenty of proteins with more than one function would be found. Unfortunately, and probably due to the difficulty in predicting a moonlighting function for a protein, few examples of bacteriophage moonlighting proteins have been extensively studied regarding their secondary function and their impact on the pathogenicity of bacteria.
The most extensively studied bacteriophage moonlighting protein is the capsid decorator protein Psu of the P4 bacteriophage. This protein has a structural role in the assembly of the viral particle by helping to stabilize the capsid. Additionally, Psu also acts as a Rho-dependent transcriptional antiterminator, interacting with the Rho factor and hence with the bacterial RNA transcription machinery. Psu therefore prevents the termination of different transcripts and controls the expression of bacterial genes that could have an impact on overall bacterial pathogenesis (Ranjan et al. 2013, 2014). Not all bacteriophage proteins that exhibit a moonlighting function in the control of bacterial pathogenicity interact directly with bacteria factors, as in the case of Psu. Given the impact of phage on bacterial growth and in horizontal gene transfer, some moonlighting functions affect the phage while indirectly impacting on bacterial pathogenicity. An example of this is found in some structural bacteriophage proteins with a lytic role, essential for the viavility of the phage (Boulanger et al. 2008; Rodriguez-Rubio et al. 2013). This is also the case for the T4 I-TevI homing endonuclease of the T4 bacteriophage that, additionally to its role in promoting intron mobility, acts as an auto-repressor of its own transcription and so controls the persistance in its host (Edgell et al. 2004).
One of the most important contributions of the bacteriophages to bacterial pathogenicity is the mobilization of other mobile genetics elements and the virulence factors that they encode. In this sense, the study of the mobilization of the Staphylococcus aureus pathogenicity islands by their helper bacteriophages has led to the identification of different bacteriophage moonlighting proteins that, apart from their role in the biology of the phage, interact with these elements and activate their transfer (Tormo-Mas et al. 2010). All of the examples mentioned previously refer to bacteriophage-encoded proteins with a dual role, interacting either with the phage, the bacteria, or with other mobile genetic elements, but there are also examples of bacteria-encoded proteins that have a moonlighting role in phage biology. The Escherichia coli thioredoxin protein that has an antioxidant function for the bacteria also moonlights as an enhancer of the replication of the T7 bacteriophage (Bedford et al. 1997; Ghosh et al. 2008). In this chapter the dual role of these proteins is described in more detail, along with their impact on the bacteriophage biology and the repercussion in bacterial pathogenicity.