nucleation

LIT-1571: How do bacterial endosymbionts work with so few genes?

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Properties5
StatusActive — read in full 2026-10-27 (NOTE-1213) from the PubMed Central copy of the version of record (PMC11020763, CC BY); worth reading as the current statement of what endosymbionts with the smallest genomes lack, envelope biogenesis below about 400 kb and parts of translation below about 200 kb, and of the host's presumed role in supplying them (THEORY-1049). Read it knowing that the thresholds come from a hand-picked matrix with draft genomes, that the composition analysis is called preliminary, and that every mechanism of host supply it names is a hypothesis.
Version3
Tagsendosymbiosis, ecology, natural-sciences
Cited as “paper” byNOTE-1213 — McCutcheon, Garber, Spencer & Warren — How do bacterial endosymbionts work with so few genes?
Cited as “source” byTHEORY-1049 — On McCutcheon and colleagues' survey, bacterial endosymbionts lose their own cell-envelope and envelope-transport genes together below about 400 kb, and begin to lose translation genes (ribosomal proteins, tRNAs, aminoacyl-tRNA synthetases) below about 200 kb, so that the smallest-genome symbionts cannot be self-sufficient and their membranes, transport and protein synthesis are presumably supplied by the host, as for organelles, THEORY-1208 — Bacterial endosymbionts lose their cell-envelope and envelope-transport genes together below about 400 kb and begin to lose translation genes below about 200 kb, and the host supplies the membranes, transport and protein synthesis that the smallest-genome symbionts no longer encode, as for organelles

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McCutcheon, J. P., Garber, A. I., Spencer, N. & Warren, J. M. (2024). How do bacterial endosymbionts work with so few genes? PLOS Biology 22(4): e3002577. doi:10.1371/journal.pbio.3002577.

Key takeaways

From the reading, NOTE-1213.

  • Two thresholds. Below about 400 kb, envelope and envelope-transport genes are lost together. Below about 200 kb, ribosomal proteins, tRNAs and aminoacyl-tRNA synthetases erode (THEORY-1049).
  • Biased proteomes. Tipping-point endosymbionts lie outside the amino-acid composition of nearly 99,000 proteomes, beyond extremophiles. Zinderia’s proteome is almost half lysine, isoleucine and asparagine. The analysis is “preliminary”.
  • The host as supplier. Three-membrane envelopes, host transporters at the symbiont vacuole, and single host-encoded proteins seen in aphid and mealybug endosymbionts suggest that the host provides what the genome lacks. The mechanisms are unknown.
  • Caveats. The thresholds come from a curated set of genomes, including drafts. The 200 kb cutoff is “somewhat arbitrary”.

Why this is here

A2 rates it “Critical”, as the complement to the 2012 review: the open mechanistic problems of extremely reduced symbionts.

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