12. Production of rRNA requires cleavage and modification events

22.11 Cleavage of the 3′ end may require a small RNA

The involvement of small RNAs in 3′ cleavage events was first detected by using an in vitro system that generates authentic 3′ ends in X. laevis histone mRNAs. When the histone H3 gene is injected into the Xenopus oocyte, it is faithfully initiated and transcribed, but termination occurs at variable sites. However, when a nuclear extract from the sea urchin is simultaneously injected with the gene, the transcribed mRNA has the proper 3′ end.

The nuclear extract has three active components: a heat-labile factor (of unknown function); a factor that binds to a hairpin just upstream of the cleavage site; and a 56 base RNA called U7 snRNA whose 5′ terminus is complementary to a short sequence just downstream of the 3′ cleavage site (Galli et al., 1983).

Figure 22.32 Generation of the 3 F end of histone H3 mRNA depends on a conserved hairpin and a sequence that base pairs with U7 snRNA.

The reaction between histone H3 mRNA and U7 snRNA is drawn in Figure 22.32. The upstream hairpin and the downstream sequence that pairs with U7 snRNA are conserved in histone H3 mRNAs of several species. The U7 snRNA has sequences towards its 5′ end that could pair with the histone mRNA consensus sequences, and has an extensive hairpin at its 3′ end.

3′ processing is inhibited by mutations in the downstream histone consensus sequence that reduce ability to pair with U7 snRNA. Compensatory mutations in U7 snRNA that restore complementarity also restore 3′ processing. This suggests that U7 snRNA functions by base pairing with the histone mRNA. Cleavage to generate a 3′ terminus occurs a fixed distance from the site recognized by U7 snRNA, which suggests that the snRNA functions at this stage (for review see Birnstiel, 1985).

Reviews

Birnstiel, M. L. (1985). Transcription termination and 3?/FONT> processing: the end is in site. Cell 41, 349-359.

Research

Galli, G. et al. (1983). Biochemical complementation with RNA in the Xenopus oocyte: a small RNA is required for the generation of 3?/FONT> histone mRNA termini. Cell 34, 823-828.