The Role of RbfA in 16 S rRNA Processing and Cell Growth at Low Temperature in Escherichia coli

RbfA, a 30 S ribosome-binding factor, is a multicopy suppressor of a cold-sensitive C23U mutation of the 16 S rRNA and is required for efficient processing of the 16 S rRNA. At 37 °C, ΔrbfA cells show accumulation of ribosomal subunits and 16 S rRNA precursor with a significantly reduced polysome profile in comparison with wild-type cells. RbfA is also a cold-shock protein essential for Escherichia coli cells to adapt to low temperature. In this study, we examined its association with the ribosome and its role in 16 S rRNA processing and ribosome profiles at low temperature. In wild-type cells, following cold shock at 15 °C, the amount of free RbfA remained largely stable, while that of its 30 S subunit-associated form became several times greater than that at 37 °C and a larger fraction of total 30 S subunits was detected to be RbfA-containing. In ΔrbfA cells, the pre-16 S rRNA amount increased after cold shock with a concomitant reduction of the mature 16 S rRNA amount and the formation of polysomes was further reduced. A closer examination revealed that 30 S ribosomal subunits of ΔrbfA cells at low temperature contained primarily pre-16 S rRNA and little mature 16 S rRNA. Our results indicate that the cold sensitivity of ΔrbfA cells is directly related to their lack of translation initiation-capable 30 S subunits containing mature 16 S rRNA at low temperature. Importantly, when the C-terminal 25 residue sequence was deleted, the resulting RbfAΔ25 lost the abilities to stably associate with the 30 S subunit and to suppress the dominant-negative, cold-sensitive phenotype of the C23U mutation in 16 S rRNA but was able to suppress the 16 S rRNA processing defect and the cold-sensitive phenotype of the ΔrbfA cells, suggesting that RbfA may interact with the 30 S ribosome at more than one site or function in more than one fashion in assisting the 16 S rRNA maturation at low temperature.