pHW121, pHW104 and pHW126 belong to different classes of plasmids replicating by the rolling circle mechanism. (A) A stretch upstream of the pHW121 repA gene is similar to replication origins of pC191/pUB110-family plasmids and the E. coli bacteriophage öX174. The experimentally determined cleavage sites of pC194 and öX174 are indicated by vertical arrows. (B) pHW104 and pHW126 are members of poorly characterised families of rolling circle plasmids. The G+C contents calculated for pAM10.6 and pM3 are based on partial sequences. (C) Evidence that pHW126 replicates via the rolling circle mechanism. Constructs containing two origins of replication of pHW126 and, as control, pHW15 were grown E. coli INVαF' for 40 generations. Subsequently DNA was isolated and analysed by restriction digestion with Hin dIII (similar results were obtained for digests with Sal I; data not shown). The expected positions of constructs containing one or two origins are indicated by arrows. The deletion of the second origin was confirmed by sequencing (data not shown). The size of the marker bands is given in kb. (D) G+C contents of small plasmids and their hosts are correlated. The trendline was calculated from 124 enterobacterial plasmid sequences retrieved from the Genome Project Database http://www.ncbi.nlm.nih.gov/genomes. For strains with unavailable genomic G+C contents the mean value of the species according to Bergey's Manual of Systematic Bacteriology  was used. Plasmids from Rahnella are shown as filled circles while plasmids from other Enterobacteriaceae are shown as open circles.