The low level expression of chloramphenicol acetyltransferase (CAT) mRNA in Escherichia coli is not dependent on either Shine-Dalgarno or the downstream boxes in the CAT gene.

Recent studies have shown that the canonical Shine-Dalgarno (SD)-anti-SD interaction is dispensable for the initiation of translation of certain mRNAs in Escherichia coli. Alternative non-SD sequences (located upstream from the initiation codon) and also downstream sequences ("downstream boxes") complementary to 16S rRNA were found to be involved in the initiation of translation of mRNAs devoid of either SD or any leader sequences. In this study the chloramphenicol acetyltransferase (CAT) gene was modified to remove the 5' terminal non-translated region and/or the two potential downstream boxes in the CAT gene. Thus a series of ten CAT gene constructs was created and expressed in E. coli under a strong constitutive promoter. The results showed that CAT mRNAs devoid of both leader sequence nucleotides and the two downstream boxes in the CAT gene remained active in vivo and produced CAT protein in sufficient amounts for survival of the transformed cells at chloramphenicol concentrations up to 20-30 micrograms/ml.