Flavin-dependent thymidylate synthase ThyX activity: implications for the folate cycle in bacteria

Area of Science:

• Microbiology
• Biochemistry
• Enzymology

Background:

• Flavin-dependent ThyX proteins exhibit thymidylate synthase activity in vitro.
• Their precise cellular functions and necessity in vivo remain unproven.
• ThyA is the canonical enzyme for thymidylate synthesis.

Purpose of the Study:

• To provide direct evidence for the cellular function of ThyX in Rhodobacter capsulatus.
• To investigate the interplay between ThyX, ThyA, and folate metabolism in thymidylate synthesis.
• To understand the impact of folate levels on bacterial growth.

Main Methods:

• Construction of a defined Rhodobacter capsulatus thyX inactivation mutant using insertional mutagenesis.
• Phenotypic analysis of the mutant strain to assess thymidylate synthesis.
• Complementation studies involving ThyA and dihydrofolate reductase (FolA).
• Mathematical modeling of bacterial folate metabolism.

Main Results:

• The Rhodobacter capsulatus thyX mutant confirmed ThyX is essential for de novo thymidylate synthesis.
• Successful complementation required both ThyA and FolA.
• Exogenous methylenetetrahydrofolate transiently inhibited growth.
• Mathematical modeling indicated low FolA activity suffices for ThyX-mediated thymidylate synthesis.
• Intracellular folate accumulation was linked to growth inhibition.

Conclusions:

• Rhodobacter capsulatus ThyX plays a critical role in cellular thymidylate synthesis.
• Bacterial folate metabolism is complex, with ThyX offering benefits under folate-compromised conditions.
• High intracellular folate levels can negatively impact bacterial growth.