Q7 - Thermus szegediensis: Nutrition and Complementation
A microbiology lab studies the microbe Thermus szegediensis, recently isolated from a thermal spring in Szeged. In a sequence of experiments (Exp), they first grew T. szegediensis under different conditions to test its nutritional requirements.
| Conditions | Exp 1 | Exp 2 | Exp 3 | Exp 4 |
|---|---|---|---|---|
| light | + | + | − | − |
| oxygen | + | + | + | + |
| nitrogen | + | + | + | + |
| phosphorus | + | + | + | + |
| thermal water salts | + | + | + | + |
| glucose | + | − | + | − |
| trace minerals | + | + | + | + |
| Growth? | yes | yes | yes | yes |
Table 1.
The lab was further interested in finding genes important for the microbe to exist at high temperatures. They isolated 6 mutants (M1–M6), then performed a complementation test between each ("+" = growth, “−” = no growth) at T = 56°C.
| M1 | M2 | M3 | M4 | M5 | M6 | WT | |
|---|---|---|---|---|---|---|---|
| M1 | − | − | + | − | + | + | + |
| M2 | − | − | − | − | − | − | |
| M3 | − | + | − | − | + | ||
| M4 | − | + | + | + | |||
| M5 | − | − | + | ||||
| M6 | − | + | |||||
| WT | + |
Table 2.
Q7.1. A microbiology lab studies Thermus szegediensis, isolated from a thermal spring. It was grown under different combinations of light and glucose (all other nutrients, including thermal water salts, always present), see Table 1 on the exam page. It grew in all four conditions, including with neither light nor glucose. Which of the following best describes T. szegediensis?
Q7.2. Which of the following does T. szegediensis most likely use as an energy source to produce complex organic compounds?
Q7.3 (M1). The lab isolated 6 mutants (M1-M6) unable to grow at 56°C and tested pairwise complementation (Table 2). A mutant that still fails to grow even when crossed with WT reveals a dominant mutation; one that grows when crossed with WT is recessive. Classify M1.
Q7.3 (M2). Classify M2 as dominant or recessive.
Q7.3 (M3). Classify M3 as dominant or recessive.
Q7.3 (M4). Classify M4 as dominant or recessive.
Q7.3 (M5). Classify M5 as dominant or recessive.
Q7.3 (M6). Classify M6 as dominant or recessive.
Q7.4. From the data in Table 2, at least how many different genes can you identify as being important for allowing T. szegediensis to exist at high temperatures?
2 or 3 (both accepted by the official key): Among the five recessive mutants, pairwise complementation groups them as {M1, M4} (fail to complement each other, same gene) and {M3, M5, M6} (all mutually fail to complement, same gene), giving 2 genes among the recessive set. M2's dominant mutation can't be placed into either group by standard complementation testing, so it's counted as a plausible third, distinct gene, hence '2 or 3' both earn credit.
Q7.5. Indicate which mutants have mutations in the same genes (based on Table 2's complementation results).
M1 and M4 share a gene. M3, M5 and M6 all share a (different) gene. M2 (dominant) and the recessive-only pairs above are the two identifiable groups.: Failure to complement ("−") means two mutations are in the same gene. M1×M4 = −, so they share a gene. M3×M5, M3×M6 and M5×M6 are all −, so M3/M5/M6 all share a (different) gene. Every other pairwise cross among the recessive mutants is "+" (complementation, i.e. different genes), consistent with exactly these two groups.
Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key