M. xanthus CRISPR3 locus. A region of 20.1 kb at the CRISPR3 locus is shown at the top, with genetic elements drawn to scale. The arrows represent ORFs, and the rectangle represents the CRISPR3 array. Upstream of CRISPR3 are seven RAMP genes: cmr1, cas10, cmr3, cmr4, cmr5 (indicated by 5), cmr6, and cas6 (indicated by 6). Downstream are six ORFs: MXAN_7275-7270 (MXAN_7272 is indicated by 72). Below is a closeup of CRISPR3. It contains 53 repeats (diamonds), 52 spacers (horizontal lines), and a leader sequence (L). The double slash indicates the omission of repeats 28 through 49. The arrowhead indicates the CRISPR3* Tn insertion in the 13th spacer. The solid arrows indicate the positions of the primers used for RT-PCR. These primers are CRISPR3_UF (UF), CRISPR3_UR (UR), CRISPR3_DF (DF), and CRISPR3_DR (DR). The open arrow indicates where CRISPR3_UR (UR) annealed in RT-PCR to produce a smaller product (shown in Fig. 5). pXY105 contains the entire CRISPR3 region, including 746 bp upstream of the first repeat and 1,542 bp downstream of the last repeat.