PHYLOGENETIC ANALYSIS OF THE ISOPENICILLIN-N-SYNTHETASE HORIZONTAL GENE-TRANSFER

A phylogenetic study of the isopenicillin-N-synthetase (IPNS) gene seq uence from prokaryotic and lower eukaryotic producers of beta-lactam a ntibiotics by means of a maximum-likelihood approach has been carried out. After performing an extensive search, rather than invoking a glob al molecular clock, the results obtained are best explained by a model with three rates of evolution. Grouped in decreasing order, these cor respond to A. nidulans and then to the rest of the eukaryotes and prok aryotes, respectively. The estimated branching date between prokaryoti c and fungal IPNS sequences (852 +/- 106 MY) strongly supports the hyp othesis that the IPNS gene was horizontally transferred from bacterial beta-lactam producers to filamentous fungi.