THE PLASMID REPLICATOR AMA1 IN ASPERGILLUS-NIDULANS IS AN INVERTED DUPLICATION OF A LOW-COPY-NUMBER DISPERSED GENOMIC REPEAT

The AMA1 sequence was isolated from a genomic library of Aspergillus n idulans on the basis of its ability to enhance transformation frequenc y and generate phenotypically unstable transformants in this fungus. T hese properties were previously shown to be the result of extrachromos omal replication of AMA1-bearing plasmids. Here we demonstrate that AM A1 is an inverted duplication of a sequence which has other isolated g enomic copies. These sequences (mobile Aspergillus transformation enha ncers, or MATEs) share a high degree of sequence similarity and exhibi t some features characteristic of mobile elements, including a potenti al Met-tRNA priming site, similar to that found in retrotransposons of the Ty-copia group, The nucleotide sequence does not encode any exten ded polypeptides but contains ARS-consensus matches and a multiply rep eated 'Spe' motif, which may be described as a symmetrically duplicate d topoisomerase I recognition site, This motif was shown to be a targe t for illegitimate recombination events, The mobility of members of th e MATE family is inferred from the observation that their chromosomal locations are highly variable between wild Aspergillus isolates. The i nverted duplication AMA1 is present in laboratory strains derived from the Glasgow isolate but not in other wild isolates tested. This indic ates that the inverted duplication AMA1 is of recent evolutionary orig in and probably does not exert any conserved function in the chromosom e. We discuss possible connections between structural features of AMA1 and its ability to promote extrachromosomal plasmid replication.