Table 2.

Iteron-mediated reduction of mini-P1 copy number and its overcoming by extra RepA in the presence of an autorepressed repA gene

Extra RepA* (source name)	Extra iterons	Copy number (stability) of mini-P1†
		incA+	ΔincA
None (pDKC426)	None	1.0   ± 0.23  (10)‡	4.2  ± 0.39 (86)
0.4× (pDKC425)	None	1.1   ± 0.15  (22)	4.7  ± 0.26 (100)
3× (pDKC424)	None	1.3   ± 0.10  (72)	5.1  ± 0.18 (100)
None (pALA96)	#9-#8 (partial)	0.39  ± 0.08§ (<1)	2.1  ± 0.09 (1.0)
0.4× (pALA178)	#9-#8 (partial)	0.54  ± 0.11§ (<1)	3.3  ± 0.25 (8.0)
3× (pALA177)	#9-#8 (partial)	0.93  ± 0.13§ (<1)	4.6  ± 0.25 (98)

^*RepA amounts are relative to the physiological level as determined previously (7). pDKC426, pDKC425, and pDKC424 are isogenic to pALA96 (10), pALA178, and pALA177 (29), respectively, except that the latter set has extra 45 bp containing iteron #9 and 15/19 bp of iteron #8. Both the iterons bind RepA in vitro (13). The latter set of plasmids thus supplied both extra iterons and extra RepA. 

^† Mini-P1 plasmids were pRJM384 (ΔrepAincA⁺) and pRJM345 (ΔrepAΔincA). All copy numbers are relative. The host was DH5Δlac(λincCrepA-lacZ). 

^‡ Numbers represent % of cells that retained plasmids after 24 hr growth on L + 50 μg/ml ampicillin plates at 37°C. The stabilities were determined as described (31). 

^§ Colony sizes on L plates containing 10 μg/ml chloramphenicol and 50 μg/ml ampicillin were small in the case of all unstable plasmids. For example in pRJM384-carrying cells, the sizes were roughly 1, 0.3, 0.4, and 0.8 in the presence of pDKC424, pALA96, pALA178, and pALA177, respectively. The copy numbers determined after drug selection are most likely overestimates in the case of unstable plasmids.