Molecular Basis for Different Levels oftet(M) Expression in Streptococcus pneumoniae Clinical Isolates

Abstract

ABSTRACTSeventy-four unrelated clinical isolates ofStreptococcus pneumoniaeharboring thetet(M) gene were studied. Seven strains with low tetracycline (Tc) MICs (0.25 to 0.5 μg/ml) were found to harbor truncatedtet(M) alleles that were inactivated by different frameshift mutations. In contrast, five strains bore deletions in thetet(M) promoter region, among which four displayed increased Tc MICs (16 to 64 μg/ml). The same promoter mutations were detected in Tc-resistant mutants selectedin vitrofrom various susceptible strains. Sequence analysis revealed that these deletions might impede the formation of the transcriptional attenuator located immediately upstream oftet(M). Expression inEnterococcus faecalisof atet(M) reporter gene transcribed from these promoter mutants conferred a level of Tc resistance similar to that observed in the parentalS. pneumoniaestrains. These results show that different levels of Tc susceptibility found in clinical isolates ofS. pneumoniaecan be explained by frameshift mutations withintet(M) and by alterations of the upstream transcriptional attenuator.