Does the epsilon sequence of phage T7 function as an initiator for the translation of CAT mRNA in Escherichia coli?

Epsilon (epsilon) sequence [UUAACUUUA, complementary to nucleotides 458-466 of the 16S ribosomal RNA (rRNA)] which is naturally occurring at the 5'-untranslated leader of phage T7 gene 10 mRNA was originally described as a powerful translational enhancer in Escherichia coli. Recent studies with this sequence led to controversial conclusions about its translational initiation and enhancing capability. In this study different sequence derivatives of epsilon were constructed to evaluate its efficiency not only to enhance translation of the chloramphenicol acetyltransferase (CAT) mRNA in E. coli, but also to function as an independent initiator of translation. It was observed that the epsilon sequence in combination with the CAT natural Shine-Dalgarno (SDn) or the SD consensus sequences enhanced, as expected, the translation of CAT mRNA. The natural epsilon sequence without an SD sequence failed to initiate or enhance the translation of CAT mRNA. However, when the complementarity of epsilon to 16S rRNA was increased from 9 to 16 nucleotides, epsilon alone (without the SD sequence) became an independent translational initiator with an efficiency of about 80% that obtained with the SD consensus sequence.