Genome organization and evolution of a eukaryotic nicotinate co-inducible pathway.

In a regulon including 11 genes (, , , , , , , , , and ) is inducible by a nicotinate metabolic derivative, repressible by ammonium and under stringent control of the nitrogen-state-sensitive GATA factor AreA and the specific transcription factor HxnR. This is the first report in a eukaryote of the genomic organization of a possibly complete pathway of nicotinate utilization. In the regulon is organized in three distinct clusters, this organization is variable in the . In some species all 11 genes map in a single cluster; in others they map in two clusters. This variable organization sheds light on cluster evolution. Instances of gene duplication followed by or simultaneous with integration in the cluster, partial or total cluster loss, and horizontal gene transfer of several genes (including an example of whole cluster re-acquisition in of section ) were detected, together with the incorporation in some clusters of genes not found in the co-regulated regulon, which underlie both the plasticity and the reticulate character of metabolic cluster evolution. This study provides a comprehensive phylogeny of six members of the cluster across representatives of all classes.