Lecture 11- Histidine operon

Histidine biosynthesis

-amino acid: can be an acid or base or base depending on micro environment

-often found in active sites of enzymes

-rare and expensive to make so its biosynthesis is tightly regulated

The histidine operon

-F’ episome: normal G enzyme, B- mutation

-Chromosome: strongly polar point mutation at G, provides low level of dehydratase enzyme inhibited by AT at B

Mutation mapping to hisO

-are constitutive (hisO^c) therefore it appears to act as an operator (a protein binding site)

-no protein was ever found to bind to hisO

-many unlinked genes can have mutations that lead to constitutive expression: hisR, hisS, hisT, hisU, and hisW

-hisR, hisS, hisU, and hisW proteins catalyze steps in the synthesis of his^tRNAhis

-hisT encodes an enzyme that modifies a U residue to pseudo U in the histRNA molecule

-mutations in these genes affect translation

DNA sequencing of his control region

-provided info and a hypothesis

mRNA secondary structures

-efficient translation of leader peptide: note ribosome position on mRNA: lots of his^tRNAhis available for translation; when ribosome is under B, has attenuator (stop signal)

-EF stem loop is a transcription termination sequence therefore transcription terminates before the hisG gene and the rest of the operon

-inefficient translation of leader peptide: note ribosome position on mRNA: different RNA stem structure; when ribosome is under A, no F, no attenuator

-formation of DE stem loop precents formation of EF loop: therefore transcription continues through the rest of the operon

-therefore, the translation rate of the leader peptide controls transcription of the his structural genes

Translation rate dependent on the concentration of _histRNA^his in the cell

-genetics evidence for the model: if the his codons in the leader peptide are changed to leu, regulation becomes dependent on leucine (replication becomes dependent on cognate amino acids)

-if a mutation is made in the E sequence that disrupts the stem loop then expression is constitutive

-if a complementary mutation is made in the F sequence attenuation is restored

-hisT slows translation