A possible explanation for the multiple polyadenylation sites in transcripts coding for a winged-bean leghemoglobin.

Five different copy DNA clones coding for the same leghemoglobin were isolated from a winged-bean (Psophocarpus tetragonolobus L.) nodule library. Although identical in sequence, they each possess a different side of polyadenylation located 93-128 nucleotides downstream of two overlapping AAUAAA putative signal sequences. By analysis of the untranslated 3' ends, a potential mRNA secondary structure can be predicted which could explain the observed polyadenylation heterogeneity. The structure is a size-variable hairpin, creating a net topological distance of 25-27 nucleotides between the canonical signal sequence and the different polyadenylation sites observed. We suggest that this type of variable secondary structure could be one among other causes that determines the apparent flexibility of plant polyadenylation. It could also confer particular properties to the mRNA in relation to stability, translation efficiency and-or nuclear export.