Enhancement of translation by the downstream box does not involve base pairing of mRNA with the penultimate stem sequence of 16S rRNA

The downstream box (DB) is a sequence element that enhances translation of several bacterial and phage mRNAs. It has been proposed that the DB enhance s translation by base pairing transiently to bases 1469-1483 of 16S rRNA,th e so-called anti-DB, during the initiation phase of translation. We have te sted this model of enhancer action by constructing mutations in the anti-DB that alter its mRNA base-pairing potential and examining expression of a v ariety of DB-containing mRNAs in strains expressing the mutant anti-DB 16S rRNA. We found that the rRNA mutant was viable and that expression of all t ested DB-containing mRNAs was completely unaffected by radical alterations in the proposed anti-DB. These findings lead us to conclude that enhancemen t of translation by the DB does not involve mRNA-rRNA base pairing.