Jumbo circular extrachromosomal elements of methane-oxidizing archaea with variably extensive metabolic and defense gene repertoires

Archaeal extrachromosomal elements (ECEs) are arguably the least well understood of all genetic elements, and few have >200 kbp (jumbo) genomes. Here, we report circular, jumbo ECEs with genomes of up to 535 kbp in length that associate with anaerobic methane-oxidizing Methanoperedens archaea. Notably, a 409-kbp genome related to jumbo ECEs is integrated into a subset of the ~4.2 Mbp Methanoperedens chromosomes at the tRNA-Asp genes. This represents the largest integrative element in Archaea and supports the jumbo ECE-host association. Multiple genome alignments and phylogenetic analyses suggest that the large ECE sizes developed by extensive DNA acquisition from Methanoperedens. The newly identified ECEs encode, and in some cases express, metabolic genes such as tetrahydromethanopterin S-methyltransferase exclusively involved in methane metabolism, and genes for nitrogen and sulfur compound transformations. Also encoded are defense systems, some of which are absent in hosts, such as hybrid Type I/Type III-A CRISPR-Cas systems. In contrast to viruses and plasmids, they have host-like replication machinery and occur at stable copy ratios of 1.44 ± 0.24:1 to the host. Overall, our results reveal a spectrum of jumbo ECEs of Methanoperedens, ranging from plasmid-like to minichromosome-like.