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

Abstract

Individually, the ribosomal proteins L1, L23, L36, and S6 are not essential for cell proliferation of Bacillus subtilis, 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 Mg²⁺ content was significantly reduced. The deletion of YhdP, an exporter of Mg²⁺, and overexpression of MgtE, the main importer of Mg²⁺, increased the cellular Mg²⁺ content and restored the formation of 70S ribosomes in these mutants. The increase in the cellular Mg²⁺ content improved the growth rate and the cellular translational activity of the ΔrplA (L1) and the ΔrplW (L23) mutants but did not restore those of the ΔrpmJ (L36) and the ΔrpsF (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 yhdP and overexpression of mgtE increased the production of Spo0A and partially restored the sporulation frequency in the ΔrplA (L1) mutant. These results indicate that Mg²⁺ can partly complement the function of several ribosomal proteins, probably by stabilizing the conformation of the ribosome.IMPORTANCE We previously reported that an increase in cellular Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ content. These findings indicate that at least part of the function of these ribosomal proteins can be complemented by Mg²⁺, which is essential for all living cells.

Keywords: Bacillus subtilis; magnesium; ribosomal protein; ribosome; ribosomes.