these proteins are represented by two enzymes, Rel Q and Rel P (18); both bacterial species lack SAHs (8).

In response to stress conditions, the activity of SASs is regulated on the transcriptional level (10, 18), as well as via activation by (p)pp Gpp (19, 20).

Moreover, all antibiotics targeting protein biosynthesis are expected to inhibit the Rel A-mediated stringent response indirectly: inhibition of translation decreases the consumption of amino acids, which leads to an increase in the t RNA aminoacylation level.

The prime example of this mechanism is seen with the antibiotic chloramphenicol, which is often used as a convenient tool for stringent response inhibition due to its fast uptake (36, 37).

These results refine our understanding of (p)pp Gpp's role in antibiotic tolerance and persistence and demonstrate unexpected drug interactions that lead to tolerance to bactericidal antibiotics.

An acute increase in (p)pp Gpp levels upon stress—the so-called stringent response—drives the reallocation of available metabolic resources, gearing up bacterial physiology for stress resistance and survival.

This regulatory system is of significant medicinal importance; (p)pp Gpp plays a key role in the regulation of bacterial virulence (2) and contributes to bacterial survival during antibiotic treatment by both increasing the antibiotic tolerance of the bacterial population as a whole (3, 4) and driving the formation of a small subpopulation of highly tolerant cells, the so-called persister cells (5–7), in a generally sensitive culture.

This antibiotic intercalates between helices 43 and 44 of 23S r RNA and the ribosomal protein L11 (31).

The latter is indispensable for the functionality of Rel A (32), while the activity of EF-G is only moderately affected by the removal of L11 (33).

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The taxonomic distribution of Rel A and Spo T is limited to , possess a single “long” bifunctional RSH, Rel (8).

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