Maturases Cooperate with Other Proteins for Splicing

Maturases are not necessarily sufficient for promoting group I intron splicing, as additional proteins are also required. The S. cerevisiae bI3 intron also requires the nuclear-encoded MRS1 protein (Kreike et al. 1986; Bousquet et al. 1990). MRS1 is homologous to the mt Ccel protein that resolves four-way DNA junctions. In vitro kinetic analyses reveal that the bI3 maturase binds the intron as a monomer while two MRS1 dimers bind cooperatively to separate sites within the intron. Binding of the maturase to its site likely occurs first and this accelerates the binding of the MRS1 dimers. Only after MRS1 binding does the intron catalyze the splicing reaction (Bassi et al. 2002; Bas-si and Weeks 2003).

The bI4 and aI4a introns require the bI4 maturase as well as the nuclear-encoded leucyl tRNA synthetase, Leu RS. In mutant strains with a disrupted bI4 maturase, a single point mutation in Leu RS activates the latent maturase activity of aI4a protein (see above). Interestingly, the location of the mutation resides in the CP1 domain of the Leu RS which functions in the editing of misactivated amino acids and binding of the tRNA acceptor helix (Li et al. 1996; Rho et al. 2002). Remarkably, the expression of the CP1 peptide is sufficient for rescuing splicing-inactivated Leu RS protein suggesting that this domain is directly involved in splicing and may recognize an intron structure analogous to the tRNA acceptor helix (Rho et al. 2002). A combination of yeast two- and three-hybrid analyses suggest that the bI4 maturase and Leu RS do not interact and bind to independent sites on the bI4 intron (Rho and Martinis 2000; Rho et al. 2002). In light of these observations, it is still unknown how the NAM2-1 mutation activates the aI4a maturase. Nevertheless, it is apparent that, even in these more complex systems, maturases function by inducing conformational changes leading to the binding of other factors and/ or RNA catalysis.

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