# Phylogenetic and functional characterization of Asgard primases

## TL;DR

This study explores the DNA replication machinery of Asgard archaea, revealing insights into their evolutionary relationship with Eukarya.

## Contribution

The paper identifies two distinct groups of Asgard primases and their structural and functional differences.

## Key findings

- Asgard primases are divided into the Heimdall and Loki groups with distinct phylogenetic and structural features.
- The Heimdall group's PriL has an extra C-terminal domain, similar to human PriL, which may stabilize an iron–sulfur cluster.
- B18_G1 primase produces short primers, a eukaryotic-like feature, unlike non-Asgard archaeal primases.

## Abstract

Eukarya resemble Archaea in DNA replication. Analysis of the DNA replication machinery of Asgard archaea may provide a valuable test of the hypothesis that this phylum is the origin of Eukarya. Among the replication proteins, primase, which comprises the catalytic subunit PriS and the non-catalytic subunit PriL, synthesizes primers for extension by DNA polymerase. Here, we show that Asgard primases fall into two major groups, denoted the Heimdall group and the Loki group, which are phylogenetically and structurally more closely related to eukaryotic primases and to primases from non-Asgard archaea, respectively. Notably, like human PriL, PriL of the Heimdall group possesses an extra C-terminal domain, which, absent in archaeal PriL of the non-Heimdall group, presumably serves to enhance the stability of the conserved iron–sulfur cluster in PriL. We overproduced and purified the PriS and PriL subunits of the Heimdall group from the Candidatus Gerdarchaeota archaeon B18_G1 in Escherichia coli. Biochemical characterization reveals that the B18_G1 primase is capable of primer synthesis and extension, preferentially using dNTPs as substrates, as shown for primases from non-Asgard archaea; however, unlike non-Asgard archaeal primases, it produces short primers, a feature typical of eukaryotic primases. These results shed significant light on the evolutionary pathway of primase and are consistent with the hypothesis of the Asgard origin of Eukarya.