Genome organization and phylogenetic distribution of a novel family of ancient murine endogenous proviruses with evidence for transposition-mediated proliferation.

A new family of murine endogenous proviruses (VL6.0) is described here. The intact provirus is near 6 kb in length and shows a genomic organization of 5' LTR, gag, pol, env, and 3' LTR. The primer binding site (PBS) is that of a tRNA(gly). The lack of functional open reading frames and occurrence of significant gaps in most, if not all, members of this group show it to be ancient. Our estimate of copy number per haploid genome is 30+. Members of this group have been isolated from Mus musculus domesticus, M. m. casteneus, M. m. hortulanus, M. caroli, and M. spretus. The occurrence of these sequences throughout such diverse members of the genus Mus may indicate that the date of the original infection predated the divergence of the extant Mus lineages at around 2.5 million years ago. Analysis of gap (deletion/insertion) patterns indicates that these sequences may have proliferated within the Mus genome by a mechanism of reverse transcriptase-mediated transposition. As yet, there are no closely related murine retroviruses described. The closest mammalian retrovirus based on sequence similarity is from the miniature swine (Sus scrofa).

A ZFY-like sequence in fish, with comments on the evolution of the ZFY family of genes in vertebrates.

ZFY-like genes have been observed in a variety of vertebrate species. Although originally implicated as the primary testis-determining gene in humans and other placental mammals, more recent evidence indicates a role(s) outside that of testis determination. In this study, DNA from five species of fish, Carasius auratus, Rivulus marmoratus, Xiphophorus maculatus, X. milleri, and X. nigrensis was subjected to Southern blot analysis using a PCR-amplified fragment of mouse ZFY-like sequence as a probe. Restriction fragment patterns were not polymorphic between sexes in any one species but showed a different pattern for each species. With one exception, Rivulus, a 3.1-kb band from the EcoRI digestion was common to all. Sequence and open reading frame analysis of this fragment showed a strong homology to other known vertebrate ZFY-like genes. Of particular interest in this gene is a novel third finger domain similar to one human and one alligator ZFY-like gene. Our studies and others provide evidence for a family of vertebrate ZFY genes, with those having this novel third finger being representative of the ancestral condition.

Conjugation of antibiotic resistance in Streptococcus suis.

Forty-eight clinical isolates of Streptococcus suis were examined for antibiotic sensitivity and the presence of plasmid DNA. It was determined that isolates from this study showed a substantial increase in resistance to erythromycin (ery), clindamycin, and tetracycline (tet) compared to a similar study conducted five years earlier. Eleven of the 48 isolates contained plasmid DNA as revealed by DNA isolation and gel electrophoresis. Plasmid DNA from four strains resistant to the above three antibiotics was tested for the ability to transform an antibiotic sensitive recipient. No transformation of antibiotic resistance could be demonstrated. In other experiments, the above four strains, along with four plasmid-negative triply resistant strains were tested for the ability to transfer tet or ery resistance to tet and ery sensitive recipients by conjugation. In each mating, antibiotic resistance was transferred at frequencies averaging 2.4 x 10(-6) recombinants/recipient for ery and 3.4 x 10(-6) recombinants/recipient for tet resistance. DNA from each clinical specimen, as well as the recombinants mentioned above was probed with tn916. Autoradiographs revealed that several clinical isolates and recombinants bound the probe. It is concluded that conjugation of antibiotic resistance in these clinical strains is possibly mediated by a transposon similar to tn916.

Structural and genetic properties of the Eb recombinational hotspot in the mouse.

The Eb gene of the mouse contains a recombinational hotspot which plays a predominant role in meiotic crossing-over within the I region of the mouse major histocompatibility complex (MHC). The nucleotide sequences of five recombinants derived from H-2k/H-2b heterozygotes at the Eb locus placed the sites of recombination in each recombinant haplotype within a 2.9 kilobase (kb) segment located fully within the second intron of the Eb gene. Further resolution of the crossover sites was not possible since the nucleotide sequences of the parental and recombinant haplotypes are identical within this segment. The molecular characterization of these five recombinants considered in conjunction with three previously reported intra-Eb recombinants indicates that there are at least two distinct sites of recombination within the Eb recombinational hotspot. In a related study, an examination of the nucleotide sequence of the H-2p allele of the Eb gene revealed a major genetic rearrangement in the 5' half of the intron in this haplotype. A 597 base pair (bp) nucleotide sequence found in the H-2p haplotype is replaced by a 1634 bp segment found in the H-2b and H-2k haplotypes. Sequence analysis of this 1634 bp segment shows strong nucleotide sequence similarity to retroposon long terminal repeat (LTR), env, and pol genes indicating that this segment of the second intron has evolved through retroposon insertion. The location of these retroposon sequences within the 2.9 kb recombination segment defined by the five H-2k/H-2b recombinant haplotypes suggests a possible relationship between these retroviral elements and site-specific recombination within the second intron of the Eb gene.

Molecular characterization of meiotic recombination within the major histocompatibility complex of the mouse: mapping of crossover sites within the I region.

The molecular analysis of crossing-over within the mouse major histocompatibility complex provides a useful approach for the study of the structural characteristics of meiotic recombination. In this study five intra-I-region recombinants, each derived from Ik/Ib heterozygotes, were characterized for restriction-fragment length polymorphisms (RFLPs) characteristic of the I region of the two parental strains. Southern blot analysis of intra-I recombinant strains A.TBR2, A.TBR3, A.TBR5, A.TBR13, and A.TBR17 using six I-region DNA probes revealed that the point of crossing-over in all five recombinants occurred within a 6.2-kb KpnI-EcoRI segment located within the E beta gene. The segments of DNA containing the crossover point from each of the recombinant chromosomes were cloned by screening partial genomic libraries constructed in lambda gt7 bacteriophage. Construction of partial restriction maps of the cloned segments from the parental and recombinant chromosomes permitted the boundaries of the area containing the crossover site to be narrowed to a 4.0-kb segment located almost entirely within an intron of the E beta gene. The recognition that the points of crossing-over in all five recombinants studied are clustered in a relatively small area of the I region provides further evidence for a hot spot of recombination associated with the E beta gene.

Usefulness of the guppy, Poecilia reticulata, in carcinogenicity testing: special advantages and problems.

The guppy, Poecilia reticulata, native to fresh and brackish waters of northeastern South America and adjacent islands of the Caribbean, has been introduced throughout much of the tropical and subtropical world. It tolerates comparatively wide ranges of temperature, salinity, and crowding. This hardiness plus high fecundity, live birth, fast growth, and early maturation make it an excellent laboratory organism for disciplines ranging from behavioral studies to toxicity testing. In 4 published carcinogenicity studies, 8 of 11 known mammalian carcinogens produced neoplasms in the liver and occasionally at other sites. The guppy's overall popularity, however, has magnified problems concerning stock origin. The natural variability of this species has been increased by artificially selected breeding and by exotic natural selection pressures of foreign environments on introduced populations. The source of many guppy stocks today is ultimately the pet trade, and a good genetic history of such strains is not often available. Differences in stock origin may account for some differential responses of growth and sensitivity to environmental variables noted in the literature. The establishment of carefully maintained inbred strains for cancer research is the best solution to this problem.