Ribosome profiling reveals an adaptation strategy of reduced bacterium to acute stress

- Mycoplasma gallisepticum demonstrate high correlation between transcription and translation under growth at normal temperature, but low under heat stress.
- Stress response of M. gallsiepticum consists of a noise-like response which envolves numerous genes and an adaptive one.
- Decrease of promoters' efficiency during the stress significantly contributes to the ribosome-bound mRNA.
- HU-protein is associated with ribosomes under growth at normal temperature but dissociates under heat stress.

Bacteria of class Mollicutes (mycoplasmas) feature significant genome reduction which makes them good model organisms for systems biology studies. Previously we demonstrated, that drastic transcriptional response of mycoplasmas to stress results in a very limited response on the level of protein. In this study we used heat stress model of M. gallisepticum and ribosome profiling to elucidate the process of genetic information transfer under stress. We found that under heat stress ribosomes demonstrate selectivity towards mRNA binding. We identified that heat stress response may be divided into two groups on the basis of absolute transcript abundance and fold-change in the translatome. One represents a noise-like response and another is likely an adaptive one. The latter include ClpB chaperone, cell division cluster, homologs of immunoblocking proteins and short ORFs with unknown function. We found that previously identified read-through of terminators contributes to the upregulation of transcripts in the translatome as well. In addition we identified that ribosomes of M. gallisepticum undergo reorganization under the heat stress. The most notable event is decrease of the amount of associated HU protein. In conclusion, only changes of few adaptive transcripts significantly impact translatome, while widespread noise-like transcription plays insignificant role in translation during stress.