Problem 8 - Biochemistry: Concepts and Connections

Question

A classic way to isolate thymidylate synthase-negative mutants of bacteria is to treat a growing culture with thymidine and trimethoprim. Most of the cells are killed, and the survivors are greatly enriched in thymidylate synthase- negative mutants. (a) What phenotype would allow you to identify these mutants? (b) What is the biochemical rationale for the selection? (That is, why are the mutants not killed under these conditions?) (c) How would the procedure need to be modified to select mammalian cell mutants defective in thymidylate synthase?

Step-by-Step Solution

Answer

(a) Thymidine auxotrophy. (b) They bypass the synthesis constraint by using external thymidine. (c) Provide thymidine and a folate metabolism inhibitor, like methotrexate, to mammals.

Step 1: Identify the Mutant Phenotype

The thymidylate synthase-negative mutants can be identified by their inability to synthesize thymidine triphosphate (TTP), an essential precursor for DNA synthesis. These mutants are auxotrophic for thymidine, meaning they depend on an external supply of thymidine to survive and grow.

Step 2: Understand the Biochemical Rationale

In the presence of thymidine and trimethoprim, normal bacterial cells are inhibited or killed. Trimethoprim inhibits dihydrofolate reductase, preventing the regeneration of tetrahydrofolate, subsequently inhibiting the synthesis of thymidine monophosphate (TMP). However, the thymidylate synthase-negative mutants survive because they can uptake thymidine from the surroundings to bypass the enzymatic step they lack.

Step 3: Modify the Procedure for Mammalian Cells

To adapt this procedure for mammalian cell selection, one must create conditions that allow only thymidylate synthase-negative mutants to survive. This could involve supplying an external source of thymidine and a folate synthesis inhibitor similar to trimethoprim, such as methotrexate. By maintaining an environment that suppresses normal cell growth due to the disruption of folate metabolism, the mutants reliant on an external thymidine supply can proliferate.

Key Concepts

Thymidine Auxotrophy

Thymidine auxotrophy refers to a condition where cells are unable to synthesize thymidine triphosphate (TTP) on their own and thus depend on an external source of thymidine for survival and growth. Thymidine triphosphate is crucial for DNA replication as it is a building block of DNA. In fact, cells that are auxotrophic for thymidine lack the ability to produce their own thymidine due to mutations, often in the thymidylate synthase enzyme. This makes them reliant on external sources of thymidine to bypass the metabolic step they are incapable of performing. Understanding thymidine auxotrophy is important for selecting bacterial mutants and studying gene functions. Thymidine is added to the growth medium so the auxotrophic mutants can survive while other cells are affected by antibiotics like trimethoprim. This process selectively enriches the medium with the desired mutants.

Biochemical Rationale for Mutant Selection

The biochemical rationale for the selection of thymidylate synthase-negative mutants relies on their unique ability to survive in a specific inhibitory environment that affects normal cells. Normally, bacterial cells synthesize thymidine monophosphate (TMP) through the action of thymidylate synthase. However, when trimethoprim is introduced, it inhibits dihydrofolate reductase, an enzyme needed to regenerate tetrahydrofolate, which is necessary for TMP synthesis. In a situation where trimethoprim is present, bacterial cells without the mutation are unable to synthesize TMP due to blocked folate metabolism. This is lethal unless thymidine is also present in the medium. Conversely, the thymidylate synthase-negative mutants can survive because they efficiently uptake thymidine from their surroundings, circumventing the need for endogenous TMP synthesis. By supplying external thymidine, these mutants can thrive while the normal cells are inhibited or killed.

Selection of Mammalian Cell Mutants

Selecting mammalian cell mutants, particularly those defective in thymidylate synthase, requires tweaking the bacterial protocol to suit more complex eukaryotic cells. Just like bacteria, mammalian cells also depend on the synthesis of thymidine for DNA replication. For a similar selection process, one needs to mimic the inhibitory conditions for mammalian cells by using inhibitors like methotrexate, which disrupts folate metabolism akin to trimethoprim in bacteria. To isolate mutants, provide an external source of thymidine along with the folate synthesis inhibitor. This setup ensures that only those mammalian cells which are thymidylate synthase-negative, and thus require external thymidine, can continue to grow. Meanwhile, normal cells are unable to thrive due to the inhibition of the endogenous folate cycle. By cultivating these mutants, researchers can study genetic and enzymatic deficiencies more accurately.