A mutant cysteinyl‐tRNA synthetase affecting timing of chromosomal replication initiation in B. subtilis and conferring resistance to a protein kinase C inhibitor.

A Bacillus subtilis mutant spnA95 was isolated as resistant at 30 degrees C to the protein kinase C (PKC) inhibitor, sphinganine, and temperature sensitive for growth. As deduced by flow cytometry measurements, the mutant has a 35% reduced initiation mass at permissive temperature, resulting in initiation of DNA replication much earlier in the cell cycle than in the wild type. This modification is accompanied by a change in cell size, as determined by phase‐contrast microscopy and flow cytometry. Therefore, this strain displays the characteristics of a novel cell clock mutant. spnA is a newly identified gene in B.subtilis and was shown to encode a cysteinyl‐tRNA synthetase. At non‐permissive temperature, the mutant was defective in the synthesis of P70, a protein with several characteristics of PKC (a cysteine‐rich protein). As one possibility, we propose that the altered timing of replication may be due to the reduced synthesis of specific cysteine‐rich proteins normally involved in controlling chromosomal replication initiation in B. subtilis.