Comparison of conserved structural and regulatory domains within divergent 16S rRNA-23S rRNA spacer sequences of cyanobacteria

Unité de Physiologie Microbienne (CNRS URA 1129), Département de Biochimie et Génétique Moléculaire, Institut Pasteur, 28 rue du Dr Roux, 75724 Paris Cedex 15, France 1 Author for correspondence: Isabelle Iteman. Tel: +33 1 45 68 84 16. Fax: +33 1 40 61 30 42. e-mail: iiteman{at}pasteur.fr PCR amplification of the internal transcribed spacer (ITS) between the 16S rRNA and 23S rRNA genes of the cyanobacterium Nostoc PCC 7120 gave three products. Two represented true ITS regions of different sizes, while the third was a heteroduplex. The longer spacer (ITS-L) contained 512 nucleotides and carried tRNA Ile and tRNA Ala genes, separated by a large stem–loop structure (V2) composed of short tandemly repeated repetitive sequences. Both tRNA genes, and the 5' half of the intervening stem, were absent from the shorter spacer (ITS-S), of length 283 nucleotides, which was otherwise almost completely identical to ITS-L. The two spacer regions of Nostoc PCC 7120 were aligned to published ITS sequences of cyanobacteria, the cyanelle of Cyanophora paradoxa and Escherichia coli . Although the ITS regions of cyanobacteria vary in length from 283 to 545 nucleotides and contain either both tRNA Ile and tRNA Ala genes, only the tRNA Ile gene, or neither, there is no correlation between ITS size and coding capacity for tRNAs. Putative secondary structures were determined for the deduced transcripts of the rrn operons of several cyanobacteria and were compared to that of E. coli . Highly conserved motifs important for folding and for maturation of the rRNA transcripts were identified, and regions homologous to bacterial antiterminators (box B–box A) were located. The conserved and variable regions of the cyanobacterial ITS are potential targets of PCR primers and oligonucleotide probes for detection and identification of cyanobacteria at different taxonomic levels. Keywords: antitermination, cyanobacterium, ITS, rRNA, transfer RNA Abbreviations: ITS, internal transcribed spacer; STRR, short tandemly repeated repetitive The GenBank accession numbers for the sequences reported in this paper are AF180968 and AF180969 for ITS-L and ITS-S, respectively.