GLUTAMINE-SYNTHETASE GENE EVOLUTION IN BACTERIA

The evolution of the prokaryotic glutamine synthetase (GS) genes, name ly the GSI and GSII isoforms, has been investigated using the second c odon positions, which have previously proven to behave as a good molec ular clock. Our data confirm the early divergence between prokaryotic and eukaryotic GSII before the splitting between plants and animals. T he phylogenetic tree of the GSI isoforms shows Archaebacteria to be mo re closely related to Eubacteria than to Eukaryotes. This finding is c onfirmed by the phylogenetic analysis carried out on both large and sm all subunits of rRNA. However, differently from the rRNA analyses, Cre narchaeota and Euryarchaeota Archaebateria, as well as high- and low-G C gram-positive bacteria, appear to be polyphyletie. We provide eviden ce that the observed polyphyly of Archaebacteria might be only apparen t, resulting from a gene duplication event preceding the split between Archaebacteria and Eubacteria and followed by the retention of only o ne isoform in the extant lineages. Both gram-negative bacteria and hig h-GC gram-positive bacteria, which appear closely related, have GS act ivity regulated by an adenylylation/deacenylylation mechanism. A later al gene transfer from Archaebacteria to low-GC eubacteria is invoked t o explain the observed polyphyly of gram-positive bacteria.