Another enzyme, DNA polymerase I, a single protein, functions in filling the gaps between Okazaki fragments. The former activity digests a short sequence of DNA, which is replaced by a radioactive strand. Gap filling is concluded by another enzyme, DNA ligase, and a phage ligase, T4 ligase, is important in connecting DNA strands in the recombinant DNA technology. Genes coding for related functions are often organized into operons. In E. coli , most of the genes are transcribed by polymerase containing Sigma 70, but more specialized genes are read by other specialized sigma factors.
Terminators are of two types.
At the highest level, transcription of many genes is controlled at once, for example by the use of different sigma factors. At an intermediate level, transcription of a set of genes, often scattered at different places on the chromosome, is regulated. But a small signalling molecule, cAMP, is made only when it is starved. Useful because these enzymes will NOT be made even when lactose is there, as long as E. coli is happily growing in a better carbon source, such as glucose. At the lowest, and most specific level, transcription of an operon is regulated by specific signals.