comA and comB are Bacillus subtilis competence genes that are identified by insertions of Tn917lac. They are classified as early genes because of their expression throughout growth; the expression of late com genes increases sharply during the transition to the stationary phase. The comA and comB determinants were cloned, and the 5' and 3' termini of their transcripts were localized by low-resolution S1 nuclease protection experiments. comA and comB were found by Southern blotting to be localized near one another, but they were nevertheless apparently transcribed independently. Epistatic relationships among the com genes were explored by using the beta-galactosidase expressed from transcriptional fusions as a marker. Late com genes were found to be dependent on the products of comA, comB, and sin for their expression. The sin gene is a transcriptional regulator that is required for the development of competence (N. K. Gaur, E. Dubnau, and I. Smith, J. Bacteriol. 168:860-869, 1986).