Pamela L. Schwartzberg

Mice homozygous for the ablm1 mutation show poor viability and depletion of selected B and T cell populations.

The c-abl gene, originally identified as the cellular homolog of the transforming gene of the Abelson murine leukemia virus, encodes a protein-tyrosine kinase of unknown function that is expressed in all mammalian tissues. We have previously described the introduction of a mutation in the c-abl gene into the mouse germline via targeted gene disruption of embryonic stem cells. We now show that mice homozygous for this mutation are severely affected, displaying increased perinatal mortality, runtedness, and abnormal spleen, head, and eye development. We have examined components of the immune system and have found major reductions in B cell progenitors in the adult bone marrow, with less dramatic reductions in developing T cell compartments.

Structure of the termini of DNA intermediates in the integration of retroviral DNA: Dependence on IN function and terminal DNA sequence

Linear retroviral DNA, the major precursor to the integrated provirus of the murine leukemia viruses, contains a mixture of two structures at its ends: some termini are full-length and blunt, and some have recessed 3 strands. A temporal study of the end structures showed that the proportion of the DNA with recessed ends increases during the course of infection, and suggests that the blunt ends are precursors to the recessed ends. We have examined the DNA structures of the ends of retroviral mutants defective in the integration (IN) function. The results show that the formation of the recessed ends requires the presence of IN. Finally, we have analyzed the structures at the ends of mutant genomes with alterations in the terminal DNA sequence. The exact position of the recessed 3 end can be recessed one, two, or four nucleotides relative to the 5 end. In all cases the position of the recessed 3 end correlates perfectly with, and thus presumably determines, the site of joining to the target DNA.

Construction and analysis of deletion mutations in the pol gene of moloney murine leukemia virus: A new viral function required for productive infection

We have used in vitro mutagenesis to explore the functions of the gene products encoded by the pol gene of Moloney murine leukemia virus (M-MuLV). Deletions were constructed at a variety of positions in the gene, and the altered DNA copies of the viral genome were introduced into mouse cells by cotransformation. The mutants could be divided into two classes depending on the phenotype and map position of the deletion within the pol gene. Mutants with deletions mapping in the 5 portion of the gene were found to be completely deficient in reverse transcriptase activity. Mutants mapping in the 3 portion of the gene, however, assembled and released virions with normal levels of reverse transcriptase and RNAase H activities. When applied to permissive cells, these virions directed the synthesis of all three forms of unintegrated viral DNA: full-length, double-stranded linear DNA and the two circular forms with one and two copies of the long terminal repeat sequences. The infection was arrested at this point and the infected cells did not become producers of virus. Thus the 3 portion of the pol gene encodes a polypeptide with a function distinct from that of reverse transcriptase, which is not required for synthesis of viral DNA but is essential for establishment of that DNA in a stable, active form in the infected cell. We suggest that this function may be the integration of the proviral DNA.