The following statistics on contaminations of pUC18 libraries with extrachromosomal elements are based on BLASTs done with reading sequences that were masked for low-complex repeats.
libs mtDNA rDNA
JAX4 abd 1.5% 6.8%
JC1 abc 0.3% 1.1%
JC2 abcdef 7.7% 2.4%
JC3 a 0.8% 1.3%