The splicing of Group I introns. This occurs by a ribozymic mechanism where the intron sequence forms a distinct 3D structure, characteristic of Group I introns and involved in determining the locations of the splice sites (there do not appear to be consensus splice site sequences) as well as having a role in catalyzing the splicing reactions, though protein factors are also required in vivo. Splicing occurs by a series of two transesterification reactions, generally with exogenous guanosine as the initiating nucleophile. The intron is excised as a linear piece (though it may subsequently circularize).

The joining together of two independently transcribed RNAs from two different genes, each of which also produces mRNA(s) via cis-splicing.

The joining together of exons from two different primary transcripts of messenger RNA (mRNA) via a spliceosomal mechanism, so that mRNA consisting only of the joined exons is produced.

OBSOLETE. Catalysis of the second transesterification reaction of U12-type spliceosomal mRNA splicing. Ligation of the two exons occurs via a transesterification reaction where the free 3'-hydroxyl group of the 5' exon is the nucleophile attacking the 3' splice site. Non-expressed sequences are now detached from the exons. In cis splicing, the intron is in a lariat structure. The catalytic site is thought to be formed by U6 and/or U2 snRNAs and/or associated proteins.

OBSOLETE. Catalysis of the second transesterification reaction of U2-type spliceosomal mRNA splicing. Ligation of the two exons occurs via a transesterification reaction where the free 3'-hydroxyl group of the 5' exon is the nucleophile attacking the 3' splice site. Non-expressed sequences are now detached from the exons. In cis splicing, the intron is in a lariat structure. The catalytic site is thought to be formed by U6 and/or U2 snRNAs and/or associated proteins.

OBSOLETE. Catalysis of the first transesterification reaction of U2-type spliceosomal mRNA splicing. The intron branch site adenosine is the nucleophile attacking the 5' splice site, resulting in cleavage at this position. In cis splicing, this is the step that forms a lariat structure of the intron RNA, which is still joined to the 3' exon. The catalytic site is thought to be formed by U6 and/or U2 snRNA, and/or associated proteins.

OBSOLETE. Catalysis of the first transesterification reaction of U12-type spliceosomal mRNA splicing. The intron branch site adenosine is the nucleophile attacking the 5' splice site, resulting in cleavage at this position. In cis splicing, this is the step that forms a lariat structure of the intron RNA, which is still joined to the 3' exon. The catalytic site is thought to be formed by U6atac snRNA and/or U2atac snRNA, and/or associated proteins.

Formation of a quadruple snRNP complex composed of the spliced leader (SL) RNA along with the U4/U6-U5 tri-snRNP complex. Interactions that may facilitate this include a duplex between the SL and U6 RNAs and interactions between the U5 RNA and the exon sequence at the 5' splice site within the SL RNA.

Assembly of a spliceosomal complex containing the intact pre-mRNA and all of the spliceosomal snRNPs. This occurs when the tri-snRNP associates with the pre-mRNA and associated snRNPs in an ATP-dependent manner.