Different effects of intron nucleotide composition and secondary structure on pre‐mRNA splicing in monocot and dicot plants.

We have found previously that the sequences important for recognition of pre‐mRNA introns in dicot plants differ from those in the introns of vertebrates and yeast. Neither a conserved branch point nor a polypyrimidine tract, found in yeast and vertebrate introns respectively, are required. Instead, AU‐rich sequences, a characteristic feature of dicot plant introns, are essential. Here we show that splicing in protoplasts of maize, a monocot, differs significantly from splicing in a dicot, Nicotiana plumbaginifolia. As in the case of dicots, a conserved branch point and a polypyrimidine tract are not required for intron processing in maize. However, unlike in dicots, AU‐rich sequences are not essential, although their presence facilitates splicing if the splice site sequences are not optimal. The lack of an absolute requirement for AU‐rich stretches in monocot introns in reflected in the occurrence of GC‐rich introns in monocots but not in dicots. We also show that maize protoplasts are able to process a mammalian intron and short introns containing stem‐‐loops, neither of which are spliced in N.plumbaginifolia protoplasts. The ability of maize, but not of N.plumbaginifolia to process stem‐‐loop‐containing or GC‐rich introns suggests that one of the functions of AU‐rich sequences during splicing of dicot plant pre‐mRNAs may be to minimize secondary structure within the intron.