In Mycoplasma hyopneumoniae, the single genes for 16S and 23S rRNAs are clustered in one operon from which the 5S rRNA gene is separated by more than 4 kbp. This operon, gene 2413 and gene X from M. hyopneumoniae and the 5' ends of both rRNA operons from M. capricolum were cloned and used for analysis of the following gene expression signals: promoters and terminators of the DNA-directed RNA polymerase, ribosomal binding sites of mRNA, and sites for processing of precursor rRNA. The analyses were performed by nuclease S1 protection experiments, the primer extension technique, and DNA sequencing. From these studies we conclude that putative promoter sequences in M. hyopneumoniae deviate significantly from those in Escherichia coli and Bacillus subtilis in having its -35 consensus sequence replaced by A-T rich sequences, whereas the characterized M. capricolum promoter resembles more closely a typical E. coli promoter. The other expression signals show sequences or structures similar to those found in other eubacteria, indicating related underlying principles.