Contributions of Zur-controlled ribosomal proteins to growth under zinc starvation conditions.

Maintaining intracellular zinc levels is critical, because zinc serves as a cofactor for many required enzymes and is toxic in excess. Bacillus subtilis Zur, a Fur family repressor, controls the zinc starvation response including two ribosomal proteins (r-proteins) paralogous to L31 and S14. Biochemical analyses suggest that Zur-controlled r-proteins (which lack the two CXXC metal-binding motifs) may functionally replace their cognate zinc-requiring proteins during zinc limitation. We demonstrate here that Zur regulates the expression of an additional r-protein paralog, RpmGC (L33c), and, using strains defective in zinc uptake, we investigate the physiological contributions of all three Zur-regulated r-proteins. In the 168 lineage, rpmGC is a pseudogene containing a frameshift mutation. Correction of this mutation allows expression of a functional L33c that can suppress the poor growth phenotype of an rpmGA rpmGB (encoding L33a, L33b) double mutant. Similarly, we provide physiological evidence in support of the "failsafe" model (Y. Natori et al., Mol. Microbiol. 63:294-307, 2007) in which the Zur-regulated S14 paralog YhzA allows continued ribosome synthesis when there is insufficient zinc to support S14 function. The L31 paralog YtiA can replace L31 and complement the growth defect of an rpmE mutant (Nanamiya et al., Mol. Microbiol. 52:273-283). We show that, under zinc starvation conditions, derepression of YtiA significantly increases the growth of cells in which preexisting ribosomes carry, as the sole L31 protein, RpmE (containing zinc), but not if they carry YtiA (which lacks zinc). These results support a direct and physiologically relevant role for YtiA in mobilizing zinc from ribosomes.