EVOLUTIONARY AFFILIATION OF THE MARINE NITROGEN-FIXING CYANOBACTERIUMTRICHODESMIUM SP STRAIN NIBB-1067, DERIVED BY 16S-RIBOSOMAL RNA SEQUENCE-ANALYSIS

The 165 rRNA sequence of Trichodesmium sp. strain NIBB 1067 was determ ined and used for the construction of a distance tree and bootstrap an alysis. The tree shows that. among the available cyanobacterial 165 rR NA sequences, Trichodesmium NIBB 1067 has Oscillatoria PCC 7515 as its closest relative, presenting 94.9% of sequence similarity with the la tter strain. This is in contrast to a difference of 9 mol% G+C in mean genomic DNA base composition between the two organisms. Nevertheless, the genotypic heterogeneity presented by a number of strains assigned to the genus Oscillatoria hinders a taxonomic decision on the separat e existence of the genera Trichodesmium and Oscillatoria. The sequence of the internal transcribed spacer (ITS) between the 16S and 23S rRNA genes was also determined, as a possible marker to study inter- and i ntraspecific variability. The ITS contains the genes coding for tRNA(l le) and tRNA(Ala) and its total length is 547 nucleotides. In six out of eight sequenced clones, there is a duplication of 29 nucleotides, s urrounding the 5' end of the tRNA(lle).