A 13-kDa protein with a helix-turn-helix motif is encoded by bacterial operons related to the SRP pathway.

We have identified a 13 kDa protein (p13) in Mycoplasma mycoides subsp. mycoides that is encoded immediately downstream of a protein homologous to E. coli FtsY, a protein taking part in the bacterial signal recognition particle (SRP) pathway. The same organisation of the p13 and FtsY genes occurs in Mycoplasma pneumoniae. PCR analysis of different mycoplasma strains revealed the same organisation in strains belonging to the Mycoplasma mycoides cluster of the mycoplasma phylogenetic tree. Searches in sequence databases identified homologues to p13 in Bacillus subtilis and Streptococcus mutans. In these bacteria the p13 protein is encoded by the same operon as a protein homologous to the 54 kDa subunit of SRP. These findings suggest that there is a functional relationship between the p13 protein and the SRP pathway. Sequence analysis of the p13 proteins strongly suggest that they have a helix-turn-helix (HTH) motif, indicating that they are gene regulatory proteins.