# Functional Analysis of Oligoadenylate Synthetase in the Emu (Dromaius novaehollandiae)

## TL;DR

The study examines the OAS genes in emus and finds they function similarly to ostriches, providing insights into antiviral defense evolution in birds.

## Contribution

The study identifies and characterizes OAS1 and OASL in emus, revealing functional similarities to ostrich OAS genes.

## Key findings

- Emu OAS1 and OASL genes show high sequence homology to ostrich OAS genes.
- Emu OAS1 has enzymatic activity, while OASL inhibits flaviviral replication.
- Emus and ostriches share similar OAS gene functions, suggesting evolutionary conservation.

## Abstract

Oligoadenylate synthetase (OAS) is a conserved antiviral protein found in several animal species. Among birds, only the ostrich has two genes, OAS1 and OASL, which show oligoadenylate synthetic activity and inhibition of flavivirus-specific genome replication, respectively. However, it remains unclear whether OAS duplication with the separated function is unique to the ostrich. Therefore, we examined the OASs of the emu, which are closely related to the ostrich. We sequenced and cloned emu OAS genes and analyzed their functions. The results showed that the two emu OAS genes had amino acid sequence homologies of 80% for OAS1 and 78% for OASL compared to those of ostriches. The amino acid sequences related to the enzymatic function were almost identical to those of ostriches. Emu OAS1 only showed OAS activity, whereas emu OASL only inhibited flaviviral replication. These results indicate that emus have characteristics similar to ostrich in terms of OAS genes. This study provides insights into the evolution of viral defense by OAS protein family in Palaeognathae.

2′-5′-oligoadenylate synthetase (OAS) is one of the proteins that act as a defense mechanism against foreign RNA in cells. OAS has two functions: an antiviral effect against a wide range of virus species via the OAS/RNase L pathway with synthesized oligoadenylates and inhibition of viral replication specific to viruses of the genus Flavivirus, which is independent of enzymatic activity. Several birds have been reported to possess only one type of OAS family member, OASL, which has both enzymatic activity and inhibitory effects on flaviviral replication. However, the ostrich has two types of OASs, OAS1 and OASL, which show different functions—enzymatic and anti-flaviviral activities, respectively. In this study, emu OASs were cloned to investigate their sequence and function and elucidate the role of OASs in emus. The cloning results showed that emus had OAS1 and OASL, suggesting that emu OASs were more closely related to ostrich than to other birds. Functional investigations showed that emu OAS1 and OASL had enzymatic and anti-flaviviral activities, respectively, similar to those of the ostrich. Emus and ostriches are evolutionarily different from most birds and may be more closely related to mammalian OAS diversity.