Translation during cold adaptation does not involve mRNA-rRNA base pairingthrough the downstream box

The downstream box (DB) has been proposed to enhance translation of several mRNAs and to be a key element controlling the expression of cold-shocked m RNAs. However, the proposal that the DB exerts its effects through a base p airing interaction with the complementary anti-downstream box (antiDB) sequ ence (nt 1469-1483) located in the penultimate stem (helix 44) of 16S rRNA remains controversial. The existence of this interaction during initiation of protein synthesis under cold-shock conditions has been investigated in t he present work using an Escherichia coli strain whose ribosomes lack the p otential to base pair with mRNA because of a 12 bp inversion of the antiDB sequence in helix 44. Our results show that this strain is capable of cold acclimation, withstands cold shock, and its ribosomes translate mRNAs that contain or lack DB sequences with similar efficiency, comparable to that of the wild type. The structure of helix 44 in 30S ribosomal subunits from ce lls grown at 37 degreesC and from cells subjected to cold shock was also an alyzed by binding a P-32-labeled oligonucleotide complementary to the antiD B region and by chemical probing with DMS and kethoxal. Both approaches cle arly indicate that this region is in a double-stranded conformation and the refore not available for base pairing with mRNA.