1 6S rRNA tRNA-Ile Ala or Glu LRNA-lle ERNA-Ala RNA region Fig. 1. Schematic representation of a 16s-23S spacer and organization of its functional regions(shadowed boxes). As indicated by brackets the presence of tRNA genes is not universal and their number and type may vary among species J. Garc a-Mart nez et alL. Journal of Microbiological Methods 36(1999)55-64J. Garc´ıa-Mart´ınez et al. / Journal of Microbiological Methods 36 (1999) 55 –64 16s rRNA