Pictures Considered #1 : Visualizing Coupled Transcription and Translation in E. coli

Some images that have appeared in the literature from time to time are truly iconic in that they tell the underlying story clearly and succinctly. We present here images that have made a difference in microbiology at the time. You are invited to suggest your choice.
 

by Elio
 

Visualizing Coupled Transcription and Translation in E. coli
 

A portion of an E. coli chromosome being transcribed (left to right) and being simultaneously translated. The arrow points to the putative site where RNA polymerase is first bound to the DNA. The chains of dark bodies are polysomes, that is, several ribosomes on the same mRNA molecule. Note that the size of the polysomes increases with distance from the upstream site where transcription began.

In 1970, before this image was published, there was indirect evidence that translation in bacteria is coupled to transcription – a pronounced departure from the eukaryote way in which one process takes place in the nucleus while, the other is relegated to the cytoplasm. This classic electron micrograph by Miller, Hamkalo, and Thomas nailed down that supposition beautifully. In the decades since, this has proven to be one of the few enduring criteria for differentiating between prokaryotes and eukaryotes.
 

Seen stretched across the first electron micrograph is a single DNA molecule. Emerging from it are a series of polysomes, each containing an increasing number of ribosomes. The interpretation is that one RNA polymerase molecule after another bound to the promoter site (arrow) and then moved from left to right, each synthesizing mRNA as it went. As each mRNA strand begins, the first of a series of ribosomes binds to the mRNA and initiates translation. As the mRNA grows, additional ribosomes are added to the polysome, each one ultimately translating the entire protein. Neither the RNA polymerase molecules nor the nascent peptides are visible, but they can well be imagined. Likewise, the putative promoter site where each RNA polymerase molecule first bound to the DNA can only be inferred, in the region upstream (to the left).
 

The 16S rRNA and 23S rRNA genes are co-transcribed. The arrow points to a region between these two genes. Intervening gene transcripts had been selectively removed by RNAse III. Direction of transcription is from left to right.

This work relied on a method developed in Miller’s lab that used a mutant E. coli strain whose cells are fragile at low ionic strength. This fragility enabled them to prepare lysates suitable for examination under the electron microscope. They went on to successfully use these methods to examine other aspects of transcription/translation in both prokaryotic and eukaryotic cells. For a review of their work, click here. One of their particularly stunning images shows the transcription of tandem 16S and 23S ribosomal RNA genes.

 
 

 
 

Comments

[Christoph Weigel]

Dear Elio, Cairn's autoradiograph showing a replicating E. coli chromosome belongs to this set of iconic pictures

http://www.evolution-textbook.org/content/free/figures/12_EVOW_Art/00_EVOW_CH12.jpg

Elio replies:

Sure. It's on the list. Many thanks.

Elio