Normal segregation of the Escherichia coli chromosome and stable inheritance of multicopy plasmids such as ColE1 requires the Xer site‐specific recombination system. Two putative lambda integrase family recombinases, XerC and XerD, participate in the recombination reactions. We have constructed an E. coli strain in which the expression of xerC can be tightly regulated, thereby allowing the analysis of controlled recombination reactions in vivo. Xer‐mediated recombination in this strain generates Holliday junction‐containing DNA molecules in which a specific pair of strands has been exchanged in addition to complete recombinant products. This suggests that Xer site‐specific recombination utilizes a strand exchange mechanism similar or identical to that of other members of the lambda integrase family of recombination systems. The controlled in vivo recombination reaction at cer requires recombinase and two accessory proteins, ArgR and PepA. Generation of Holliday junctions and recombinant products is equally efficient in RuvC‐ and RuvC+ cells, and in cells containing a multicopy RuvC+ plasmid. Controlled XerC expression is also used to analyse the efficiency of recombination between variant cer sites containing sequence alterations and heterologies within their central regions.

No abstract

SUMMARYThe genomic DNAs of bovine papillomavirus (BPV) type 1, type 2 and type 4 were cloned in pAT153. BPV 1 and BPV2 genomes were cloned using the single HindlII sites of the vector and virus DNAs, and BPV4 was cloned using the single BamHI sites. The orientation of the recombinant DNAs was established by restriction enzyme digestion, hybridization and heteroduplex analysis. The results showed that: (i) BPV 1 and BPV2 DNAs are in register and are broadly homologous throughout most of their length when aligned at their single HindlII sites; (ii) depending on the degree of hybridization stringency used, the two DNAs show one major region and several minor regions of partial homology, mainly residing in the segment of the genomes believed to contain the structural genes; (iii) BPV4 DNA shares no homology with either BPV1 or BPV2 DNA.

The myeloproliferative sarcoma virus (MPSV) was derived by passage of Moloney sarcoma virus (Mo-MuSV) in adult mice. Mo-MuSV variants transform fibroblasts. However, MPSV also affects erythroid, myeloid, and hematopoietic stem cells. The MPSV proviral genome, two temperature-sensitive mutants derived from it, Mo-MuSV variant M1, and Moloney murine leukemia virus (Mo-MuLV) were compared by heteroduplex mapping. MPSV wild type was found to have 1 kilobase pair deleted from the pol gene and to contain v-mos-related sequences. The 3' end of MPSV, including the oncogene-helper junctions, the v-mos gene, and the 3' long terminal repeat, was sequenced and compared with sequences of Mo-MuLV, MSV-124, and the mouse oncogene c-mos. From these data, MPSV appears to be either closely related to the original Mo-MuSV or an independent recombinant of Mo-MuLV and c-mos. Five possible explanations of the altered specificity of MPSV are considered. (i) The MPSV mos protein has properties inherent in c-mos but lost by other Mo-MuSV mos proteins. (ii) The MPSV mos protein has altered characteristics due to amino acid changes. (iii) Due to a frameshift, MPSV codes for a mos protein truncated at the amino terminal and also a novel peptide. (iv) A second novel peptide may be encoded from the 3' env region. (v) MPSV has long terminal repeats and an enhancer sequence more like Mo-MuLV than Mo-MuSV, with a consequently altered target cell specificity.

Recently metamorphosed female Xenopus laevis toads were injected with tritiated thymidine .Animals were kept at 20°C and were sacrificed 1-23 days after isotope injection . Radioautographs of squash preparations of the ovaries were made . The progress of labeled germ cell nuclei was followed to obtain information on the time course of early meiosis and extrachromosomal DNA synthesis . Premeiotic S was estimated to take not more than 7 days .Leptotene takes 4 days, zygotene takes 5 days, and pachytene was estimated to be completed in about 18 days . The major period of amplification of the extrachromosomal DNA occurs in pachytene and takes about 13 days . A low level of synthesis was observed before and after this period, in zygotene and late pachytene-early diplotene, extending the total time for extrachromosomal DNA synthesis during meiosis to about 18 days . These data allowed the calculation to be made that one round of replication of the amplified DNA takes between 1 .2 and 3 .0 days . It was also found that in both oogonial and premeiotic interphases, the nucleolus-associated DNA shows asynchronous (probably late) labeling with respect to the chromosomes .