Magnesium Suppresses Defects in the Formation of 70S Ribosomes as Well as in Sporulation Caused by Lack of Several Individual Ribosomal Proteins.

Individually, the ribosomal proteins L1, L23, L36, and S6 are not essential for cell proliferation of , but the absence of any one of these ribosomal proteins causes a defect in the formation of the 70S ribosomes and a reduced growth rate. In mutant strains individually lacking these ribosomal proteins, the cellular Mg content was significantly reduced. The deletion of YhdP, an exporter of Mg, and overexpression of MgtE, the main importer of Mg, increased the cellular Mg content and restored the formation of 70S ribosomes in these mutants. The increase in the cellular Mg content improved the growth rate and the cellular translational activity of the Δ (L1) and the Δ (L23) mutants but did not restore those of the Δ (L36) and the Δ (S6) mutants. The lack of L1 caused a decrease in the production of Spo0A, the master regulator of sporulation, resulting in a decreased sporulation frequency. However, deletion of and overexpression of increased the production of Spo0A and partially restored the sporulation frequency in the Δ (L1) mutant. These results indicate that Mg can partly complement the function of several ribosomal proteins, probably by stabilizing the conformation of the ribosome. We previously reported that an increase in cellular Mg content can suppress defects in 70S ribosome formation and growth rate caused by the absence of ribosomal protein L34. In the present study, we demonstrated that, even in mutants lacking individual ribosomal proteins other than L34 (L1, L23, L36, and S6), an increase in the cellular Mg content could restore 70S ribosome formation. Moreover, the defect in sporulation caused by the absence of L1 was also suppressed by an increase in the cellular Mg content. These findings indicate that at least part of the function of these ribosomal proteins can be complemented by Mg, which is essential for all living cells.