The SSV1 viral integrase is not essential.

Viral integration is a widely conserved characteristic in viruses in all domains of life; however, its necessity is not well understood in many cases. Integration using tyrosine recombinases is one of the most widespread and best characterized mechanisms of integration. We completely removed the tyrosine recombinase integrase from the hyperthermophilic and acidophilic archaeal virus SSV1 using a novel LIPCR technique and found that the virus still replicated and spread in its host Sulfolobus solfataricus without integration. The mutant virus maintained a persistent infection but the integrase-lacking virus was less competitive than the wild-type virus when co-cultured. Based on these results, we discuss the necessity of integration and the possible advantages of this type of replication strategy.