How Many Maturases

In total, genetic studies in S. cerevisiae identified three dedicated maturases in the cob gene designated bI2, bI3, and bI4. In each case, the maturase facilitates splicing of its cognate intron while the bI4 maturase also promotes splicing of the homologous aI4a intron in the cytochrome oxidase subunit I (COXI) gene (reviewed in Lambowitz et al. 1999). Interestingly, the aI4a intron also encodes a latent maturase protein that is activated by a single mutation in its ORF (miml-1) or by a dominant suppressor mutant (NAM2-1) of an additional splicing co-factor, the nuclear encoded mt leucyl tRNA synthetase (Leu RS, see below; Du-jardin et al. 1982; Lambouesse et al. 1987; Herbert et al. 1988). Outside of S. cerevisiae, four other maturases have also been discovered. The bI2 intron maturase from S. capensis differs by only four amino acids from the corresponding maturase from S. cerevisiae and, not surprisingly, the S. capensis maturase also functions in splicing (Szczepanek and Lazowska 1996). The first intron in the mt cytochrome c oxidase (coxlll) from S.pombe encodes a maturase (Merlos-Lange et al. 1987). Finally, the protein (I-Anil) encoded by the single intron (A.n. COB) in the cob gene of Aspergillus nidulans can facilitate the splicing of the A.n. COB intron in vitro, as can the related maturase from Venturia inaequalis (Ho et al. 1997; Kwon and Waring, unpubl.; Table 1). Although other maturases most certainly exist, studies with this handful of intron-splicing systems have revealed significant insights with regard to the function of these unusual proteins.

Table 1. Maturases confirmed by experimental data

Species Gene Introna Maturase reference

Endonuclease Endonuclease reference

S. cerevisiae cyt. b

S. cerevisiae cyt. b

S. cerevisiae cyt. b bI2

V. inaequalis cyt. b

oxidase subunit I

VibI5

Lazowska et al. (1980)

Lazowska et al. (1989)

Anziano et al. (1982); de la Salle et al. (1982); Weiss-Brummer et al. (1982)

Kwon and

Waring

COX1I1 Merlos-Lange et al. (1987)

No, unless two Szczepanek mutations and Lazowska made

No, but stimulates recombination in E. coli

Goguel et al. (1989)

Yes, I-Spol Pellenz et al.

S. capensis cyt. b

A. nidulans cyt. b

S. cerevisiae cyt.

oxidase subunit I

Szczepanek and Lazowska (1996)

Yes, I-ScflI Lazowska et al. (1992)

aI4a Dujardin with et al. (1982) mim2-\ mutation

Yes,I-SceII

a Strains can vary in the total number of introns and whether the indicated intron is present.

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