Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology

The tRNA Leu (UAA) intron has been recorded in the plastid genome of many algae and land plants and was the first intron to be discovered in cyanobacteria. In all known cases it interrupts the tRNA Leu anticodon loop at a conserved position (U-intron-AA). Cyanobacteria are a diverse group of photosynthetic prokaryotes, some involved in symbiotic associations with a wide range of organisms. The most studied associations are those with plants, where strains of Nostoc are the common cyanobacterial partner. In this thesis two aspects of the biology of the cyanobacterial tRNA Leu (UAA) intron are focused: first, the use of the intron as a genetic marker for studying the diversity and specificity of two cyanobacterial symbiosis (bryophytes and cycads) and second, the evolutionary patterns of the intron by using the unique data set generated from the diversity analysis. From the studies, many different Nostoc strains are involved in the two symbiotic associations, although no variation was observed within a single bryophyte cavity or cycad coralloid root. Furthermore, a certain level of temporal stability in the cyanobiont composition of the bryophyte population was found and, in the cycad association different coralloid roots from a single specimen may harbor different cyanobacteria. That a minor cyanobiont could have avoided detection is still possible but unlikely. The sequence alignment of the Nostoc tRNA Leu (UAA) introns reveals great sequence similarity with size variation only found in the structural element P6b. This element was found to consist of heptanucleotide repeats and of other non-repetitive genetic elements (NIS elements). The sporadic occurrence of the NIS elements indicates recent origins and a mechanism for its dispersal is proposed. In this thesis new insights are given concerning cyanobacterial symbioses and also on the mechanisms involved in the evolution of an old genetic element: the tRNA Leu (UAA) intron in cyanobacteria. Härtill 5 uppsatser Diss. (sammanfattning) Uppsala : Uppsala universitet, 2004 degree of Doctor of Philosophy filosofie doktorsexamen Doctor philosophiae Docteur ès sciences Lecture Hall, room 3041, Department of Physiological Botany, Villavägen 6, Uppsala Härtill 5 uppsatser Diss. (sammanfattning) Uppsala : Uppsala universitet, 2004 filosofie doktorsexamen Doctor philosophiae The tRNA Leu (UAA) intron has been recorded in the plastid genome of many algae and land plants and was the first intron to be discovered in cya