Plasmid immunity assays of Mtb Csm in heterologous host Escherichia coli. (A) C43 harbouring pCsm1–6 and pCRISPR are transformed with pRAT (control plasmid) or with pRAT-target (plasmid with pUC19 MCS target). Target transcription was arabinose-inducible. In the absence of arabinose, the transformation efficiency was independent of the presence or absence of target; whereas when target transcription was induced, a 2-log reduction of transformation efficiency was observed for the target plasmid compared to the control plasmid. *** P-value (Welch two sample test, two tailed) < 1e-05; significance threshold at < 0.05. (B) C43 harbouring pCsm1–6 or indicated mutants and pCRISPR_TetR (target tetracycline resistance gene, constitutively expressed) were transformed with pRAT. Vector control (Ctrl): backbone vectors without inserts; WT Csm, Csm1–5 only (ΔCsm6, C6), Csm3 D35A (C3), Csm1 (Cas10) D630A/D631A (cyclase mutant, Cy), Cas10 H18A/D19A (HD mutant, HD).