Extended Data Table 1a. Adaptation experiments with E. coli BL21-AI cells.

Following overnight growth with or without induction of Cas1+2 cloned on pWUR plasmid, the CRISPR array was amplified and sequenced to determine the fraction of arrays that acquired a new spacer. Results with BL21-AI with an empty pWUR vector (without Cas1+2) are presented as a control.

Sample	rep	# of reads spanning the CRISPR array	# of reads supporting unmodified parental array	# of reads supporting acquisition of a new spacer	% expanded arrays
BL21-AI, no ara	1	25,718	25,163	555	2.16%
BL21-AI, no ara	2	32,807	31,800	1,007	3.07%
BL21-AI, 0.2% ara	1	28,188	17,438	10,750	38.14%
BL21-AI, 0.2% ara	2	33,973	21,843	12,130	35.70%
BL21-AI, empty vector, no ara	1	12,021	12,021	0	0%
BL21-AI, empty vector, no ara	2	14,729	14,729	0	0%
BL21-AI, empty vector, 0.2% ara	1	28,251	28,251	0	0%
BL21-AI, empty vector, 0.2% ara	2	6,827	6,827	0	0%