I think the identification of regulatory elements, primarily promoters, which are specific DNA regions responsible for transcription initiation. Muskan Gupta

First many phage promoters have evolved to be very strong promoters because phage proteins need to be made rapidly and in large amounts during a phage replication cycle. Secondly, many phage promoters like T3, T7 and SP6 use their own polymerase and so are independent of the host polymerase and this permits high level transcription not in competition with host genes.