Using the organomercuric compound 3,6-bis(acetatomercurimethyl)dioxane in conjunction with CS2 S04 density gradient equilibrium centrifugation a (dG+dC)-rich DNA fraction constituting 10% of the ovine genome was separated from the remainder. Further fractionation allowed four distinct classes of DNA to be identified with buoyant densities in neutral CsCI of 1· 714, 1· 717, 1 ·725 and 1·716 g/cm 3 • Computerized curve resolving of the data indicated the presence of several additional DNA classes.Data obtained using restriction endonuclease digestions indicated that the 1·714 g/cm 3 satellite consists principally of an 820-base pair (bp) tandemly repeating unit. The 820-bp DNA is also present in the 1· 717 and 1· 725 g/cm 3 satellites but as a minor component. The principal components of the 1·717 and 1·725 g/cm 3 fractions are 125-, 176-and 235-bp fragments. In addition, these fractions contain 705-bp tandemly repeated material. Two or possibly three species of 22-bp tandem repeats were found only in the 1·725 g/cm 3 DNA.

DNA from each of two specialized transducing lambda phage, lambda dcysJIHD and lambda cysJ, has been analysed by heteroduplex mapping. The segment of the Escherichia coli chromosome carried by lambda dcysJIHD was shown to be large, approximately 18 kb in length, and to replace a large length of lambda DNA, approximately 11 kb, which includes the genes for integration and recombination. Thus lambda dcysJIHD is a bio-type transducing phage. lambda cysJ was shown to have lost very little lambda DNA and to carry about 8 kb of bacterial DNA. Sites for several restriction endonucleases were mapped in the DNA from each phage and cloning experiments located some of the genes of the cluster in relation to the restriction map. Cysteine regulation of the cloned cysJ and cysD genes was shown as well as cysteine regulation of beta-galactosidase in some constructs. The direction of transcription of the cysD gene was established, and from physical evidence the size of the 'silent section' between the cysH and cysD genes was estimated to be at least 11 kb.