Question

Bacterial operons & regulating metabolic pathways

Two kinds of control: enzyme regulation vs. gene regulation. Why?

trp operon. How does the presence of tryptophan prevent the synthesis of more tryptophan (2 different ways)?

lac operon. Compare & contrast with trp.

Why are operons for catabolic and anabolic pathways regulated differently?

Positive vs. negative gene regulation. Why would both be used for the same operon?

AraC mechanism. Is AraC an inducer or a repressor?

Answer 1

Gene regulation is the informal term used to describe any mechanism used by a cell to increase or decrease the production of specific gene products like proteins or RNA. Cells can modify their gene expression patterns to trigger developmental pathways, respond to environmental stimuli or adapt to new food sources. Any step of gene expression may be modulated-- from the DNA-RNA transcription step to post-transcriptional modifications of a protein.

As for enzymes, the binding molecules control whether an enzyme is activated or inhibited. They may change the shape of the active site so that substrates cannot bind to them. They can even make their active sites more receptive to substrates. Enzymes are also regulated by feedback mechanism.

The trp operon is a group of genes that is used or transcribed together to code for the components for production of tryptophan. The trp operon is regulated in such a way that when tryptophan is present in the environment, the genes for tryptophan synthesis are not expressed.

Lac operon can be activated by a chemical, allolactose, while the trp operon can be inhibited by the chemical tryptophan.

The trp operon contains five structural genes - trp A, trp B, trp C, trp D and trp E. these genes encode tryptophan synthetase. It also contains a repressive regulator gene called trp R which has a promoter where RNA polymerase binds and synthesizes mRNA for a regulatory protein. This protein now binds to the operator which then causes the transcription to be blocked.

In the lac operon, allolactose binds to the repressor protein, allowing gene transcription while in the trp operon, tryptophan binds to the repressor protein, thus blocking the gene transcription. However, in both cases, repression is that of RNA polymerase transcribing the genes in the operon.

Unlike the lac operon, the trp operon contains a leader peptide and an attenuator sequence which allows for graded regulation.

Catabolism is the breakdown of complex molecules in living organisms to form simpler ones. These simple molecules are then utilized by the body for various metabolic processes and release of energy.

Anabolism is the reverse process of catabolism. In this , the body uses simple molecules to synthesize various complex molecules needed by the body, like, synthesis of proteins in the cells from amino acids. So, the operons for the two pathways are regulated differently.

Gene regulation works through various factors and feedback mechanism is one important way to regulate them.

Cytarabin or araC is used as a medication in cancer treatment. It combines a cytosine base with an arabinose sugar and acts as an antimetabolic agent. It interferes with the synthesis of DNA. Its mode of action is due to its rapid conversion into cytosine arabinoside triphosphate , which damages the DNA when the cell cycle is in the S-phase [ this is the phase when DNA is synthesized and chromosomes are replicated]. Rapidly dividing cells, which require DNA replication are most affected. Cytosine arabinoside also inhibits both DNA and RNA polymerases and nucleotide reductase enzyme needed for DNA synthesis.

So, AraC is a repressor because it inhibits DNA synthesis and cell division besides repressing other cellular functions.