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.