Ribosomes are the genetic Turing machines that translate nucleic acid into protein. And fast growing bacteria need plenty of them. E coli bacteria, for example, contain some 73000 ribosomes per cell.

Given that E coli populations double every 20 minutes, new ribosomes have to be created at a fantastic rate. The process requires ribosomal RNA to be built at a rate of 68 transcripts per minute compared to a typical rate of 10 per minute for messenger RNA.

That requires a huge density of RNA polymerase shuttling around to build the ribosomal RNA and the question is: how do bacteria manage it without generating life throttling traffic jams?

Stefan Klumpp and Terence Hwa at the University of California, San Diego, say it looks as if these cells have built in traffic cops whose sole job is to keep the traffic moving.

The problems arise when RNA polymerase pauses for whatever reason. Pauses are thought to be necessary for proper transcription but they often occur for no good reason, a problem known as antitermination. This causes severe traffic jams.

To prevent this, Klumpp and Hwa say that a termination factor called Rho seems to unblock these jams by removing prematurely paused RNA polymerase and replacing it with new polymerase, thereby restarting traffic. A bit like emergency services removing broken down vehicles from a highway.

The researchers have developed a model to describe this process which includes studies of the behaviour of single molecules in vivo.

Interesting stuff and puzzling too. It suggests that Rho actually enhances transcription rather than attenuates it, which is counterintuitive for a termination factor.

Ref: arxiv.org/abs/0811.3163: Stochasticity and Traffic Jams in the Transcription of Ribosomal
RNA: Intriguing Role of Termination and Antitermination

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