BioTechniques News
Beatrice Bowlby
A teeny-tiny marine microbe with one of the smallest genomes ever discovered is pushing the limits of what it means to be alive.
An international team, including researchers from the University of Nottingham (UK) and the University of Tsukuba (Japan), has identified a minuscule microorganism with an extraordinarily compact genome – the smallest ever reported for an archaeon. Despite having just 238 kilobase pairs (kbp), Candidatus Sukunaarchaeum mirabile, as the novel species has been named, appears to be capable of replication, as well as gene and protein expression, demonstrating a remarkable capacity for survival with very little genetic material.
Extreme genome reduction such as this has been documented before, usually among endosymbionts whose hosts compensate for lost functions; earlier this year, for example, a genome as small as 50 kbp was reported in an ancient bacterial pathogen. In archaea, a domain of life that is similar to but evolutionarily distinct from bacteria, the previous smallest known genome belonged to Nanoarchaeum equitans and measured a miniature 490 kbp (for reference, the human genome is roughly 3.1 million kbp). However, lifeforms with the smallest genomes typically lose their identity as an autonomous cell, and it remained a mystery as to whether an organism could shrink its genome to such an extent without entirely surrendering its ability to operate independently.
The scientists behind the latest study stumbled upon an answer to this question by happy accident, while analyzing the genome of another marine microorganism, Citharistes regius. Single-cell genome amplification of bacteria associated with this microorganism revealed a genome with just 238 kbp – less than half the size of the smallest known archaeal genome – and only 189 protein-coding genes.
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Although it has lost virtually all recognizable metabolic pathways, the diminutive microbe, Candidatus Sukunaarchaeum mirabile, has retained core machinery for DNA replication, transcription and translation. “This suggests that Sukunaarchaeum may be close to the minimum level of genetic information needed for an organism to remain an independent cell,” Thorsten Allers (University of Nottingham), one of the study’s co-authors, surmised.
Interestingly, it also dedicates around a quarter of its genome to large membrane proteins, the functions of which are unknown. This is commonly seen in parasitic archaea, which may suggest that it too is a pathogenic organism.
In addition, phylogenetic analyses demonstrated that Sukunaarchaeum is evolutionarily very distinct and cannot be placed within any recognized phylum. Meanwhile, environmental surveys indicated it likely belongs to a diverse, geographically dispersed and previously overlooked clade.
Many questions still surround the mysterious little microbe – it hasn’t even been directly observed – so the team hope to continue to investigate it, and, in doing so, further our understanding of the minimum genetic requirements for an organism to function.
“The key to defining life is whether something can replicate itself, and whether it can do this autonomously,” Allers concluded. “This exciting discovery provides new clues about how simple a living cell can become while remaining capable of reproducing and maintaining its own genetic information.”
The post Pushing the limits of life: mysterious microbe has record-breaking tiny genome appeared first on BioTechniques.
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