Lecture 15: Gene Regulation in Prokaryotes

I.Importance

- Timing of gene function is crucial to efficient cell and organism function

- Basic understanding of relatively simpler genome of prokaryotes must precede understanding of eukaryote gene regulation

II. Transcription and Translation in Prokaryotes

RNA polymerase participates in all 3 phases of transcription: initiation, elongation, termination

Translation occurs during transcription (no nuclear membrane) and bacterial genomes are polycistronic

III. Timing of regulation of gene expression

How much of a particular polypeptide is present at a particular time in the bacteria?

1.binding of RNA polymerase to the promoter

2.shift from transcriptional initiation to elongation

3.release of mRNA at termination of transcription

4.stability of mRNA after synthesis

5.efficiency of ribosomes to recognize different translational initiation sites

6. stability of the polypeptide product

Most important is the binding of RNA polymerase to DNA at the promoter, the rest is fine-tuning

IV. A model system: lactose utilization in E. coli

Advantages: rare mutants could be isolated and mapped

Induction: a specific molecule stimulates production of a given protein, the molecule responsible is the inducer

The PaJaMo experiment

isolation of lac- mutants (unable to metabolize lactose)

two loci: lacZ (B-galactosidase) and lacY (permease)

constitutive mutants: both proteins produced even in absence of lactose, due to mutations in lacI

Repressor bound to inducer cannot bind to DNA

Repressor not bound to inducer binds to the DNA of the operator

inducer causes reversible changes in conformation of repressor

allosteric- change in conformation of protein when bound to another protein

Experimental evidence (Fig 16.9)

General Rule:

If a gene encodes a diffusible substance (usually a protein), it will be dominant and act in trans. Alters a protein.

If a gene affects only the expression of adjacent genes on the same DNA, it is cis acting. Alters a DNA binding

site

Why do E. coli growing in a medium of glucose and lactose not initiate lac gene transcription??

Catabolite repression

Conclusion: Regulation of lac operon depends on both negative and positive regulation

V. A different regulatory system- Arabinose operon

AraC protein has two conformations- mutation to AraC- indicate positive regulation (no expression)

VI. A different regulatory system- Tryptophan operon

Attenuation

VII. Summary

A variety of gene regulation mechanisms have evolved for efficient use of resources by prokaryote cell,

utilizing both positive and negative regulation and further fine-tuning

Terms/Concepts to know: induction, inducer, operator site, allosteric protein, operon theory (details), exptl proof (Fig 16.9), positive and negative regulation, lac, ara and tryp operon

Figs and Tables: 16.3, 16.5, 16.6, 16.7, 16.8, 16.9, 16.10, 16.11, 12, 16.16, 16.17, 16.18, 19, 20, T16.1