Presence of multiple group I introns closely related to bacteria and fungi in plastid 23S rRNAs of lichen-forming Trebouxia

The chloroplast-encoded large subunit ribosomal RNA gene of several free-living green algae contains group I\udintrons at Escherichia coli genic positions 1917, 1931, 1951, and 2449. Herein we report the presence of group I introns at these\udpositions within the chloroplast-encoded large subunit ribosomal RNA gene of several lichen-forming green algae belonging to\udthe Trebouxia genus. In contrast to the introns inserted at position 2449, all introns inserted at positions 1917, 1931, and 1951\udcontained LAGLIDADG homing endonuclease genes. Phylogenetic analyses show that: (i) introns inserted at positions 1917,\ud1931, and 1951 are closely related to introns located at homologous insertion sites in bacterial rDNA genes; and (ii) introns inserted\udat position 2449 are closely related to fungal introns located at homologous insertion sites in mitochondrial rDNA genes. The\udsymbiogenetic thalli of some lichens are proposed as the likely setting of horizontal transfer of genetic material among distantly\udrelated organisms such as bacteria, fungi, and green algae.Francisco.Gasulla@uv.es; Eva.Barreno@uv.e