MODEL & EVIDENCE · REQUIRED BY THE BOOKS
Beyond Chapter 14:
a cell with its virome
The atlas transcribes the catalogue of X-ray of Life, Vol. III, Chapter 14. Elsewhere, the same books require defence, regulation and maintenance proteins that the catalogue leaves out. They are listed here, each with its page.
How this relates to the 1,051. This page records what the books require. The minimal count includes defence against viruses, but only the smallest complete system that does the job: a CRISPR-Cas system of 9 proteins. How every protein in the 1,051 was chosen is explained on How the working minimum was reached.
The minimal unit is a cell with its virome
Every known cell lives with viruses, and viruses cannot exist without cells. The Cell: An Unsolved Mystery concludes that “the minimum unit of biological life is a cell-virome ecosystem, not a cell” (pp. 260, 322). X-ray of Life reaches the same point from the cell’s side: “Early Earth conditions would have contained abundant viral and parasitic genetic elements”, and “any proto-cell lacking adequate defense mechanisms would have been quickly overwhelmed by viral predation or parasitic genetic elements” (Vol. III pp. 231–234). The first life forms are also described as a community exchanging genes (Vol. I pp. 404–406).
The atlas therefore models the minimal cell together with its virome, living in fluctuating natural conditions: pH changes of ±2 units, temperature swings of up to 40 °C and osmotic swings of up to 1000 mOsm (Vol. III p. 509). A laboratory minimum is not the natural minimum: the 473-gene JCVI-syn3.0 cell “is a laboratory artifact, not a natural cell” (The Cell: An Unsolved Mystery, p. 325).
Defence against viruses and foreign DNA
Chapter 14 lists only “Cellular defense (3 enzymes), 900 aa” (Vol. III p. 505), which is the book’s fallback for “simpler defense systems in early life” (p. 245). Chapter 4 says the defence systems “appear to be foundational rather than derived features, suggesting they were necessary from the very beginning” (p. 232), and the closing summary repeats that “early cellular life required sophisticated defense mechanisms”, totalling over 13,462 aa (pp. 769–770).
| System (book’s example proteins) | Book size | Where |
|---|---|---|
| Type I restriction-modification: HsdM ×2, HsdR ×2, HsdS (R2M2S1 complex) | 3,602 aa | Vol. III pp. 233–235 |
| Type II restriction-modification: endonuclease ×2, methyltransferase, type IIS enzyme ×2. “Likely present in the earliest cellular life forms.” | 1,369 aa | Vol. III pp. 236–237 |
| Type III restriction-modification: Mod and Res Corrected: the book adds a separate “Type III DNA helicase” and counts three copies of each subunit (6,063 aa). The helicase is a domain of Res, and the complex is Mod₂Res₁ or Mod₂Res₂, about 2,240–3,200 aa. | ≈2,240–3,200 aa | Vol. III pp. 239–240 |
| Type II toxin-antitoxin: VapC ×2, VapB ×2, regulator ×2. Abortive infection protects the population from viral predation. | 604 aa | Vol. III pp. 242–243 |
| CRISPR-Cas: Cas1 ×2, Cas2 ×2 and a nuclease. “Likely present in early cellular life.” Corrected: the 984-aa Cas9 is from Campylobacter jejuni, not Streptococcus pyogenes (1,368 aa). Cas9 belongs to type II CRISPR, found only in bacteria; types I and III occur in both bacteria and archaea. Your paper Forty Hallmarks of Design in the Cell calls CRISPR “not a core origin requirement”, so its place here awaits your decision. | 1,824 aa (as printed) | Vol. III pp. 232–233 |
| Simpler defence (already in Chapter 14): RNase, methyltransferase, endonuclease | 900 aa | Vol. III p. 245 |
| Integrase, listed with the first-life DNA proteins: “interactions between primitive cellular life and viral entities might have been prevalent” | – | Vol. II p. 441 |
| DNA methyltransferase “protecting against foreign DNA”; enzymes that degrade foreign genetic material | – | Vol. II pp. 462, 487 |
Restriction-modification is also the cell’s first epigenetic memory: methylation marks its own DNA as self (The Cell: An Unsolved Mystery, pp. 232–233, 246–248).
Regulation and sensing
Chapter 14 has “Sigma factors (4 proteins)”, “Primary sigma factor σ70” and “Transcription factors (1 protein)” (Vol. III p. 501). Volume II gives a larger minimum: 12–18 kinds of transcription factor and 3–4 sigma factors for a cell of about 1,300 genes (pp. 504–505).
| Required regulators | Why | Where |
|---|---|---|
| σ70, σS (RpoS), σ32 (RpoH), σ54 | Housekeeping, stationary phase and stress, heat shock, nitrogen | Vol. II pp. 509–510 |
| RpoH-like, RpoS-like and Lrp-like regulators | “To adapt to thermal fluctuations, environmental stresses and nutrient availability” | Vol. II p. 509 |
| Transcription factors such as LexA (DNA damage), FNR (oxygen and redox), Fur (iron) | Examples from the book’s minimal set | Vol. II pp. 504–505 |
| Two-component systems: histidine kinase and response regulator; PhoR-PhoB “from life’s earliest stages” | Sensing the environment | Vol. II pp. 493, 499, 627; Vol. III p. 118 |
| Stringent response: RelA, SpoT, DksA; ribosome hibernation: RMF, HPF | Starvation | Vol. II pp. 590–591; The Cell pp. 282, 286 |
| Small-RNA regulation: Hfq, RNase E, RNase III | “Critical even in minimal cellular systems” | Vol. I pp. 342–343 |
Proteins needed from the first generation
| Protein | Why | Where |
|---|---|---|
| Inorganic pyrophosphatase | Removes the pyrophosphate released by DNA, RNA and protein synthesis; without it all three stall | The Cell p. 243 |
| tmRNA and SmpB | Rescue ribosomes stalled on broken messages | Vol. III pp. 274, 303; The Cell p. 198 |
| Glycyl-tRNA synthetase | Missing from the book’s synthetase list, which has 18 of the 20 | Vol. II; Vol. III p. 502 |
| Photolyase | Repairs UV damage “from the beginning” | Vol. II p. 529 |
| Reverse gyrase | Protects DNA in a hyperthermophilic cell | Vol. II (hyperthermophile machinery) |
| pH-sensing two-component system | Switches on acid-consuming enzymes | The Cell p. 238 |
| Heat-shock control: σ32, FtsH, DnaK | “A constitutive component of the minimum viable cell” in a hot environment | The Cell pp. 244–246 |
Supporting literature
- Krupovic, M., Dolja, V. V. & Koonin, E. V. (2020). The LUCA and its complex virome. Nature Reviews Microbiology 18, 661–670. Cited in X-ray of Life, Vol. III p. 752.
- Weiss, M. C. et al. (2016). The physiology and habitat of the last universal common ancestor. Nature Microbiology 1, 16116.
- Ouzounis, C. A. et al. (2006). A minimal estimate for the gene content of the last universal common ancestor: exobiology from a terrestrial perspective. Research in Microbiology 157, 57–68. 1,006–1,189 gene families (Vol. I p. 389).