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 introns at Escherichia coli genic positions 1917, 1931, 1951, and 2449. Herein we report the presence of group I introns at these positions within the chloroplast-encoded large subunit ribosomal RNA gene of several lichen-forming green algae belonging to the Trebouxia genus. In contrast to the introns inserted at position 2449, all introns inserted at positions 1917, 1931, and 1951 contained LAGLIDADG homing endonuclease genes. Phylogenetic analyses show that: (i) introns inserted at positions 1917, 1931, and 1951 are closely related to introns located at homologous insertion sites in bacterial rDNA genes; and (ii) introns inserted at position 2449 are closely related to fungal introns located at homologous insertion sites in mitochondrial rDNA genes. The symbiogenetic thalli of some lichens are proposed as the likely setting of horizontal transfer of genetic material among distantly related organisms such as bacteria, fungi, and green algae (AU)

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Changes in the photosynthetic apparatus of diatoms in response to low and high light intensities

The centric diatom Cyclotella cryptica and two strains of the pennate diatom Phaeodactylum tricornutum were grown under low and high light intensities (300 lux and 3,000 lux) over 4-6 weeks. Growth was monitored by repetitive cell count. The culture media were replaced weekly to avoid morphological and biochemical alterations caused by nutrient depletion. The ultrastructure of the cells was examined by transmission electron microscopy. Alterations in the light-harvesting antenna systems were investigated by Western immunoblotting. Both diatoms reduced the plastid area, i.e. decreased the amount of thylakoid lamellae, under high light intensity. The thylakoids still ran in groups of three with parallel orientation within the chloroplasts. The girdle band lamellae were not affected. The amounts of storage compounds and vacuoles increased. SDS-PAGE of total cell protein followed by Western immunoblotting with antisera directed against subunits of the light-harvesting antenna systems of C. cryptica (cc-antiserum) and the cryptophyte Cryptomonas maculata (cmac-antiserum) revealed that both diatoms reduced the amount of antenna polypeptides under increased light intensity. The cc-antiserum immunodecorated two bands with relative molecular masses (Mr) of 18,000 and 22,000 in C. cryptica. Both decreased under high light conditions to 67.2 +/- 6.1%. Five to seven bands in the Mr range of 14,000-27,000 were recognized in P. tricornutum. They decreased to 83 +/- 5.3%. Furthermore, the immunolabeling pattern for both strains differed under the two light regimes. The cmac-antiserum immunodecorated two polypeptides with Mr of 24,000 and 23,000 in C. cryptica, while both strains of P. tricornutum had five polypeptides in the Mr range of 14,000-24,000 that showed some differences in staining intensities between the two strains and in response to the light intensity applied (AU)

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