Trans-splicing

Some genes are in unconnected pieces, requiring RNA splicing to assemble a complete mRNA.

Chlamydomonas is facultatively photoauxotrophic. It can be grown in the dark provided a carbon source, as well as in the light without a carbon source.
A mutant with a large deletion in its chloroplast DNA is unable to grow in the light. In the light, it can't make psa, a component of the photosynthetic machinery.
The deletion does not include the psa gene which is in 3 parts (two parts in one orientation and the third in the opposite orientation) in the chloroplast DNA genome.
Analysis of psa RNA suggests a failure of RNA splicing.
Transformation of Chlamydomonas' chloroplasts by particle bombardment using cloned subfragments of wild type plastid DNA from the deleted region identified a small region that had the ability to complement the mutation.
The minimal complementing region produces a single RNA transcript whose ends were mapped.
The RNA could be translated into a few very small peptides. Yet, insertion of in frame stop codons did not block the ability of the fragment to complement.
Parts of the RNA sequence are complementary to the 3' flanking sequences of exon 1 and others to the 5' flanking sequences of exon 2.

Trans-splicing occurs between separate RNAs, the products of different transcription units.
At least in this case, trans-splicing requires a trans-acting factor. That factor is an RNA molecule.
The role of the RNA appears to be to base pair with each of the two exon-containing molecules to form a structure resembling a group II intron. A typical group II intron has six stem loop structures.
The group II intron consensus secondary structure does not require loops in some stem loop regions for activity.

Several other plastid RNAs are also assembled by trans-splicing.
A nuclear-encoded protein is required in A. thaliana mitochondria for trans-splicing around one intron of the nad1 gene (ref).

E-mail inquiries to U. Melcher------------Last Updated: 12 December, 2007