The selection-induced His+ reversion in Salmonella typhimurium

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• Spontaneous reversion to His+ in Salmonella typhimurium hisG46 occurs during histidine starvation.
• Previously, no pre-existing His+ revertants were observed in rich media.
• Increased cell concentration influences the observation of pre-existing His+ revertants.

Purpose of the Study:

• To investigate the conditions and mechanisms underlying adaptive His+ reversion in Salmonella typhimurium.
• To determine the role of DNA replication and specific genetic factors in this reversion process.
• To examine the effect of histidine deprivation on mutation rates and the emergence of different types of revertants.

Main Methods:

• Culturing Salmonella typhimurium strains (hisG46 and hisG428) under conditions of histidine starvation and varying cell concentrations.
• Monitoring His+ reversion events over time using plating techniques.
• Assessing DNA synthesis during starvation using incorporation assays.
• Investigating the role of specific genes (recA, mucAB) and inhibitors (chloramphenicol, novobiocin) on reversion and DNA synthesis.

Main Results:

• Pre-existing His+ revertants of hisG46 are observed at high cell concentrations (10(10) cells/ml).
• Selection-induced His+ reversion begins after 2-3 hours of histidine starvation and continues for approximately 4 days.
• Significant DNA synthesis occurs during the initial hours of starvation, inhibited by chloramphenicol and novobiocin.
• Adaptive His+ reversion is recA-independent but requires RecA activity on a mucAB genetic background.
• Histidine starvation does not increase the general mutation rate but induces intragenic revertants and ochre suppressors for hisG428.

Conclusions:

• Adaptive His+ reversion in Salmonella typhimurium under histidine starvation is primarily driven by DNA replication.
• The process is recA-independent, highlighting a distinct pathway for adaptive mutations.
• Histidine deprivation can specifically induce certain types of mutations, including suppressors, without broadly increasing the mutation rate.