tion fails
Genes are copied into messenger RNA by the large, complex molecular machine known as RNA polymerase II (RNAPII). RNAPII works by separating the two strands of the DNA helix opening what's called a "transcription bubble" -- and moving along the strand to be transcribed like an engine along a cog railway, copying the genetic information in the gene to create a length of messenger RNA as it goes.
As the RNAPII engine trundles along, enclosing the DNA and the open bubble, it sometimes encounters a damaged nucleotide on the strand it is transcribing -- the sort of damage that might have been inflicted by ultraviolet radiation or oxidative damage, for example. Or it may encounter situations such as a highly repetitive sequence of bases, with folding of the DNA upon itself, which make progress difficult. Unable to continue copying, RNAPII pauses and sometimes grinds to a halt.
Exposure to sunlight causes decreased transcription when ultraviolet-induced DNA damage blocks RNAPII. Transcription-coupled repair normally removes the blocking damage and allows transcription to recover; in Cockayne Syndrome, however, transcription cannot resume, and the affected cells die.
"It has been assumed that defects in transcription-coupled repair are the cause of Cockayne Syndrome, although the question remained whether was the problem was inability to deal with damaged DNA or a subtle defect in general transcription," says Cooper. "To understand the exact nature of the problem we first needed to know how stalled RNAPII is recognized."
The protein CSB, which in a mutated form causes one kind of Cockayne Syndrome, was known to recognize stalled RNAPII; CSB was presumed to be involved in initiating transcription-coupled repair, but by what means was unknown. In addition, there were hints for an important but unknown role for the protein XPG in transcription-coupled repair. In both humans and mice, when gene mutations produce a severely t
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Source:DOE/Lawrence Berkeley National Laboratory
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