Molecular analysis of the translational attenuator of a constitutively expressed erm(A) gene from Staphylococcus intermedius

During the course of a study on macrolide, lincosamide and streptogramin B resistance among staphylococci from animal sources, a Staphylococcus intermedius isolate was found to carry a constitutively expressed erm(A) gene on the 70 kb plasmid pSES29. The molecular basis of constitutive erm(A) expression was investigated by cloning and sequence analysis of the erm(A)-associated translational attenuator. Two point mutations in this regulatory region were detected. These mutations cause constitutive erm(A) gene expression by destabilization of mRNA secondary structures required for the inducible type of erm(A) gene expression.