Regulatory regions of ColE1 that are involved in determination of plasmid copy number.

The copy number of plasmid ColE1 has been known to increase when the Hae II-C segment downstream from the replication origin is deleted. The presence of the 306-base-pair (bp) Hpa II region in the segment is sufficient for reduction of the copy number. Plasmids harboring the region express a trans-acting function that is responsible for the copy number reduction and synthesize a unique protein. A protein specified by the region is purified to near homogeneity and identified as the 63-amino-acid protein encoded by the Hpa II segment. The region, which includes segments 19-25 bp and 53-311 bp downstream from the start site of the primer RNA, is involved in determination of sensitivity to the inhibitory function. In vivo transcription of the galK gene, which is directed by the primer promoter in a segment inserted in front of the structural gene, is inhibited by a plasmid carrying the Hpa II region. The inhibition is strong when the promoter segment contains up to 135 bp downstream from the primer RNA start site, whereas it is weak when only the region up to 52 bp downstream is present.