FIG 6.

Model for the roles of Hfq and RNase R cooperation in RNA quality control. Hfq and RNase R are required for stable RNA quality control and degradation. Both enzymes are required for the normal turnover of tRNA molecules and are key factors in a novel rRNA quality control mechanism that eliminates abnormal rRNA fragments from the cell. Notably, no other major exoribonuclease seems to be involved in this rRNA degradation pathway. Upon Hfq and RNase R inactivation, unprocessed 17S and pre-23S rRNAs highly accumulate in the cell, and the levels of intact 70S ribosomes are strongly reduced. This correlates with a marked increase in the population of free 30S and 50S subunits that are unable to associate. These findings strongly suggest the existence of severe defects in the ribosome biogenesis process in the absence of Hfq and RNase R.