In Prokaryotes transcription and translation are coupled -- that is, ribosomes begin translation while the mRNA is being produced.
Coupled transcription and translation. During transcription RNA polymerase (RNAP) separates the two DNA strands and forms a short region of base pairing between the coding strand of DNA and the newly synthesized RNA. As the RNA elongates it is displaced as the single-stranded DNA rehybridizes to form double-stranded DNA. Ribosomes rapidly bind to the resulting single-stranded RNA and begin protein synthesis.
Rho dependant polarity. The process of Rho dependant polarity is shown stepwise in the cartoons below. Rho dependent polarity is usually initiated by premature transcription termination.
When ribosomes encounter a stop (aka "nonsense") codon, they fall off of the RNA.
Rho factor cannot bind to RNA coated with ribosomes, but Rho factor binds to specific regions in naked RNA.
Rho factor then interacts with RNAP, causing it to fall off of the DNA which terminates transcription. Thus, when translation termination occurs within a gene it can cause transcriptional termination, preventing expression of downstream genes. This process is called translational polarity.
Rho dependant polarity is clearly more complex than the model shown above. There is biochemical and genetic evidence that Rho factor interacts with other transcription termination factors (for example, Nus) and the alpha subunit of RNAP.
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Last modified December 20, 2005