Restriction maps of the cDNA copies of the genomic segments of Φ6, L, M, and S as well as diagrams of the chimeric constructions used in this study. The sequences are all embedded in plasmid pT7T319U and the transcripts produced by T7 RNA polymerase have the correct base sequence at the 5′ ends. pLM1725 was prepared by ligating the 5′ end of segment S cut at the EcoRV site to a blunted SgrAI site of segment L. The aim was to prepare a transcript with the pac site of S, containing no sequence from M, but having a size close to that of the sum of S and M. pLM1827 was prepared by ligating segment M cut at the PstI site to segment S cut at the BamHI site to form an MS chimera (pLM1822) and then inserting the complete lacZ gene from plasmid 1871 into the ApaI site of M with PstI linkers. The aim was to prepare a transcript with the pac site of M, containing no sequence from L, but having a size close to that of the sum of M and L. pLM1809 contains the sequence of the three segments in the order S, M, L with the addition of a 427-base lacZα fragment to attain the size of the complete genome. The aim was to prepare a transcript with the pac site of S that was of the same size as the total Φ6 genome.