I am currently cloning codon-optimized I-SceI gene (from S. cerevisiae) into pET vector using E. coli DH5a, but I am keep failing cloning. Even if I assume a leaky expression of I-SceI cloned into pET vector (harboring T7 expression system) in E. coli DH5a strain--that I do not understand, either--I do not understand this phenomenon as there is no I-SceI recognition site in DH5a, as I understand. Is there any special factor required for cloning I-SceI gene in E. coli?