Native and recombinant capping enzymes block transcription in vitro.

Tempst/Elsevier

The transcription apparatus barely gets started before shutting itself down, according to Lawrence Myers (Dartmouth Medical School, Hanover, NH), Paul Tempst (Memorial Sloan-Kettering Cancer Center, New York, NY), and colleagues.

The authors have found that the two yeast mRNA capping enzymes, Cet1p and Ceg1p, are potent in vitro inhibitors of transcription. Cet1p, stimulated by its Ceg1p binding partner, stalled transcription by blocking reinitiation at already activated promoters. Conversely, elimination of Cet1p in vivo stimulated transcription. Although Cet1p is the first general repressor found to act at reinitiation, this step is well suited for transcription inhibition. “If 100 transcripts are made from a promoter,” says Myers, “only one is made at initiation—the remaining 99 are made by reinitiation.”

A block by the capping enzyme could ensure that RNA polymerase does not make more transcripts than can be properly processed. It is still unclear how capping enzymes block reinitiation. Possibly, the interaction of Cet1p with proteins in the reinitiation scaffold may prevent further polymerase subunits from entering. The inhibition must be lifted to allow multiple rounds of transcription. Here, Myers speculates that the final step of capping (methylation, which does not occur in the in vitro system) might release repression and allow reinitiation. ▪

Reference:

Myers, L., et al.
2002
.
Mol. Cell.
10
:
883
–894.