Translational block to expression of the Escherichia coli Tn9-derived chloramphenicol-resistance gene in Bacillus subtilis

Abstract

The Gram-negative product-encoding Tn9-derived chloramphenicol-resistance (Cmr) gene can be cloned but not phenotypically expressed in Bacillus subtilis. We show that, even when transcribed from B. subtilis promoters, the ribosomal binding site for the Cmr gene does not function well in B. subtilis. The Cmr gene product, chloramphenicol acetyltransferase (CmAcTase; acetyl-CoA:chloramphenicol 3-O-acetyltransferase, EC 2.3.1.28), is detected in B. subtilis when the promoters, ribosomal binding sites, and initiation codons of B. subtilis genes are fused to the Cmr gene. These gene fusions lead to the in vivo production of mRNAs containing B. subtilis translation start signals followed in an open reading frame by the translation start site normally used by Escherichia coli to initiate translation of Cmr mRNA. Both fusion and native CmAcTase proteins are produced in E. coli, but only fusion CmAcTase is produced in B. subtilis. We conclude that the absence of native CmAcTase in B. subtilis is due to inability of the E. coli ribosomal binding site to function well in B. subtilis. Since fusion CmAcTase polypeptides are produced in E. coli, we conclude that these particular B. subtilis regulatory elements function heterologously in E. coli. The absence of a suitable binding site on the Cmr gene for B. subtilis ribosomes is consistent with reports that many E. coli genes are not expressed in B. subtilis and that E. coli mRNA functions poorly in B. subtilis in vitro translation systems. The functioning of B. subtilis regulatory sequences in E. coli is consistent with in vivo and in vitro data showing the expression of B. subtilis genes in E. coli. To confirm the hypothesis that the large CmAcTase proteins are NH2-terminal fusions of native CmAcTase we partially determined the sequence of one CmAcTase fusion protein.