Constitutive expression of erythromycin resistance mediated by the ermAM determinant of plasmid pAMβ1 results from deletion of 5′ leader peptide sequences

We have sequenced the erythromycin resistance determinant (erm) of the Streptococcus faecalis plasmid pAMβ1 to investigate its relationship to other known resistance determinants. We show that this determinant is strongly (99%) homologous at the DNA level to that of plasmid pAM77 (Streptococcus sanguis) and of transposon Tn917 (S. faecalis). Moreover, nucleotide sequence comparison with the determinants of pAM77 and Tn917 shows that most of the probable regulatory region is absent, providing an explanation for the constitutive expression of the pAMβ1 erm determinant.