'boxA'-like sequence between the 16 S/23 S spacer in rRNA operon of mycoplasmas.

We have found that a boxA-like sequence is conserved in the 16 S and 23 S rRNA intergenic spacer regions of mycoplasmas, and that it always locates on loop regions of the hypothetical secondary stem-loop structures. A nucleotide sequence similar to the '-10' box of prokaryotic promoters was identified at upstream sites of the boxA-like sequence in the 16 S/23 S spacer regions. These structures may represent an internal promoter between the 16 S and 23 S rRNA genes in mycoplasmas.