Vi­su­al­iz­ing Cou­pled Tran­scrip­tion and Trans­la­tion in E. coli

Pic­tures Con­sid­ered #1

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 in our new se­ries Pic­tures Con­sid­ered 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

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.

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.

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.

(Added in 2026) For the con­nois­seurs, here is the orig­i­nal elec­tron mi­cro­graph at high res­o­lu­tion and with­out ar­rows and scale:

Vi­su­al­i­sa­tion of a gene in­volved in pro­tein syn­the­sis: trans­mis­sion elec­tron mi­cro­graph (TEM) of a struc­tural gene operon from the bac­terium Es­cherichia coli, show­ing the cou­pled tran­scrip­tion of DNA into mes­sen­ger RNA (mRNA) mol­e­cules and the trans­la­tion of this mes­sen­ger code into pro­tein mol­e­cules. This trans­la­tion of mRNA is pro­tein syn­the­sis, and takes place on the dense gran­u­lar ri­bo­somes (dark cir­cles) that ap­pear in this im­age. Mag­ni­fi­ca­tion: x160,000 at 8x10-inch size. Credit Prof. Os­car L. Miller / Sci­ence Photo Li­brary. Source