The dualrole of a cyanophage thioredoxin
Marinecyanobacteria are highly abundant photosynthetic organisms that contributegreatly to global primary production. Their viruses, marine cyanophages, arealso extremely abundant and have a major effect on cyanobacterial populations,diversity and evolution. One abundant family of cyanophages, the T7-likecyanopodoviruses can be divided into two major clades, A and B, which areclosely related but differ significantly in their infection dynamics, as wellas in their abundance and distribution. Here, we set out to test whether thethioredoxin gene, found only in clade A phages, contributes to the differencesbetween the clades. We hypothesized that it is involved in phage DNAreplication similar to the role of the host gene in the T7-E. colisystem. To test our hypothesis, we used a newly developed gene inactivationsystem to study the thioredoxin gene, trxA in the clade A phages Syn5.We found that the deletion of this gene caused a decline in phage DNAreplication. To our surprise, we also found that trxA expression inhibitsthe growth of the cyanobacterial host. Our work revealed how a non-essential host-acquiredgene contributes in more than one way to cyanophage fitness. Furthermore, ourresults indicate that some of the physiological differences between the twoclades of the T7-like cyanopodoviruses can be attributed to a clade-specific gene.