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FIGURE 4.3 Convergent nuclear gene expression in kinetoplastids and dinoflagellates. In both kinetoplastids and dinoflagellates, genes (gray) may be arranged in tandem arrays (Top) that are expressed on a single polycistronic mRNA (Middle). This mRNA is broken into multiple monocistronic mRNAs concomitantly with trans-splicing, which adds a short, capped spliced leader sequence (black) to the 5′ end of all monocistronic mRNAs. In kinetoplastids, the genes (A–C) are arranged randomly and are functionally unrelated, but, in dinoflagellates, the coding sequences on a single polycistronic mRNA are tandem repeats of 1 gene.

FIGURE 4.3 Convergent nuclear gene expression in kinetoplastids and dinoflagellates. In both kinetoplastids and dinoflagellates, genes (gray) may be arranged in tandem arrays (Top) that are expressed on a single polycistronic mRNA (Middle). This mRNA is broken into multiple monocistronic mRNAs concomitantly with trans-splicing, which adds a short, capped spliced leader sequence (black) to the 5′ end of all monocistronic mRNAs. In kinetoplastids, the genes (A–C) are arranged randomly and are functionally unrelated, but, in dinoflagellates, the coding sequences on a single polycistronic mRNA are tandem repeats of 1 gene.

addition, at the 5′ end, of an SL already equipped with a methylated cap, followed by the polyadenylation at the 3′ end (Campbell et al., 2003).

Far less is known about the organization of dinoflagellate genomes. Due to the enormous size of their nuclear DNA, nearly all sequencing of dinoflagellate genes performed to date has focused on expressed sequence tags, which do not provide information on the context of the gene. Nevertheless, what little is known about dinoflagellate genomes suggests a fascinating parallel with kinetoplastids. It now appears that some genes are isolated in the genome, but others are organized as tandem repeats (Bachvaroff and Place, 2008). These gene repeats are cotranscribed, resulting in polycistronic messages, and different from those of kinetoplastids because mRNAs have so far only been found to carry multiple copies of a single gene (Bachvaroff and Place, 2008). These transcripts are apparently processed into monocistronic mRNAs, which are presumably the substrates for trans-splicing.

In kinetoplastids, the presence of polycistronic mRNAs, together with the absence of introns, is frequently argued to be an ancient holdover,



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