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.
Visualizing Coupled Transcription and Translation in E. coli
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).
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.