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 li brary. Although identical in sequence, they each possess a different s ide of polyadenylation located 93-128 nucleotides downstream of two ov erlapping AAUAAA putative signal sequences. By analysis of the untrans lated 3' ends, a potential mRNA secondary structure can be predicted w hich could explain the observed polyadenylation heterogeneity. The str ucture is a size-variable hairpin, creating a net topological distance of 25-27 nucleotides between the canonical signal sequence and the di fferent polyadenylation sites observed. We suggest that this type of v ariable secondary structure could be one among other causes that deter mines the apparent flexibility of plant polyadenylation. It could also confer particular properties to the mRNA in relation to stability, tr anslation efficiency and-or nuclear export.