Structural alterations in the translational attenuator of constitutively expressed ermC genes

Sequence deletions of 16, 59, and 111 bp as well as a tandem duplication of 272 bp with respect to the corresponding sequence of pT48 were identified in the regulatory regions of constitutively expressed ermC genes. Constitut ive ermC gene expression as a consequence of these structural alterations i s based on either the prevention of the formation of mRNA secondary structu res in the translational attenuator or the preferential formation of those mRNA secondary structures which do not interfere with the translation of th e ermC transcripts. A model for the development of sequence deletions in th e ermC translational attenuator by homologous recombination is presented an d experimentally tested by in vitro selection of constitutively expressed m utants in staphylococcal strains deficient and proficient in homologous rec ombination.