Excision of a conjugative transposon in vitro by the Int and Xis proteins of Tn916

The roles of purified Int and Xis proteins of the conjugative transposon Tn 916 in excision of a deletion derivative of the closely related element Tn 1545 were investigated. At a low salt concentration (37.5 mM NaCl), Int alone was able to promote limited excision to produce a covalently closed circular form of the transposon, showing that Tn 916 Int can catalyze both DNA cleavage and strand exchange. This reaction was stimulated by Xis. At higher salt concentrations (150 mM NaCl), excision by Int alone was reduced to barely detectable levels and Xis was required for excision. The low salt, Xis-stimulated reaction was approximately 8-fold more efficient than the high salt, Xis-dependent reaction. These results reflect in vivo requirements for Int and Xis in excision.

Molecular characterization of Mls-1

Recently a series of endogenous and exogenous superantigens have been described which have one common feature, namely, they lead to in vivo deletion and in vitro stimulation of T cells expressing particular T cell receptor V beta genes. The Mls antigens represent the prototypes of these molecules. We have mapped Mls-1 to the endogenous mammary tumor virus (MMTV) Mtv-7, while other SAG have also been associated with various MMTV. The open reading frame gene of the MMTV encodes the SAG. Thus, the new terminology MMTV sag has been proposed for this gene. Transfection experiments suggest that the expression of MMTV sag is tightly controlled, probably by a negative acting factor encoded within the open reading frame. Furthermore, a pronounced IL-4 effect is seen in the functional detection of the transfected Mtv-7 sag. Since this lymphokine does not influence the mRNA level of the endogenous or transfected MMTV genes, it is likely that it exerts its effect by increasing transcription of MHC class II genes, whose products are required for functional detection of Mls. We have identified one mouse strain, MA/MyJ, which has an Mls-1 phenotype but does not contain Mtv-7. The SAG activity of this strain was mapped to a new mammary tumor provirus, Mtv-43, not seen in other inbred strains. Sequence analyses revealed that the predicted amino acid sequences of the Mtv-7 and the Mtv-43 sag genes are very similar. This is particularly striking in the C-terminus, where all other MMTV sag sequences differ 100%. Thus, this region of the molecule seems to control the V beta specificity of SAG molecules. It is likely that the SAG expression provides an advantage for the infectious MMTV, probably by facilitating its transmission by T cells from the site of primary residence in the gut to its final destination, the mammary glands.

Linkage of Mls genes to endogenous mammary tumour viruses of inbred mice

T cells that recognize self antigen are clonally deleted in the thymus--a maturation process that occurs in the context of histocompatibility molecules and the T-cell receptor. The minor lymphocyte stimulation antigens (Mls) effect these deletions through interactions with the V beta portion of the T-cell receptor, thus mimicking bacterial 'superantigens'. Intrigued by the fact that each known Mls gene maps to the same chromosomal region as an endogenous mouse mammary tumour virus (Mtv), we reevaluated the linkage relationships between the two gene families. Here we report perfect concordance in inbred and recombinant inbred mice between the presence of four Mtv proviruses with the expression of Mls gene products. These data suggest a general model in which mammary tumour virus gene products themselves are the ligands that shape a considerable portion of the immunological repertoire of common laboratory mice.

Circulating prostacyclin and thromboxane levels in patients with diabetic retinopathy

Prostacyclin (PGI2) is the most potent endogenous inhibitor of platelet aggregation yet discovered. Thromboxane (TXA2) promotes aggregation and degranulation of platelets. It is hypothesized that an homeostasis exists between these pathways that is protective against vascular damage and is disturbed in several diseases such as diabetes. Circulating levels of PGI2-TXA2 in 35 patients with adult onset diabetes and 15 controls have been assayed. Twenty patients had background retinopathy, and 15 had proliferative retinopathy. Circulating levels of PGI were found to be elevated in 9/15 patients with proliferative diabetic retinopathy, 2/20 diabetic patients with background or no retinopathy, and 0/15 controls. PGI levels may correlate, therefore, with the severity of the retinopathy.

Fibrinogen Houston: a dysfibrinogen exhibiting defective fibrin monomer aggregation and alpha-chain cross-linkages

A 38-year-old male patient with a life-long history of easy bruising and mild bleeding had a prolonged activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT). Reptilase (Bathrops atrox) clotting time was normal. His undiluted and diluted plasma prolonged the APTT, PT, and TT of normal plasma. Fibrin produced from patient plasma was insoluble in 5 M urea. Plasma fibrinogen level was increased when measured as clottable protein and by Laurell electroimmunoassay. Specific assays of plasma factors II, V, VII, X, VIII, IX, XI, and XII were normal. A circulating antithrombin in patient plasma was excluded by demonstrating normal thrombin-induced platelet aggregation of gel-separated platelets in the presence of patient plasma. Purified patient fibrinogen reproduced the anticoagulant effect of patient plasma. Patient fibrinogen antigen was similar to normal fibrinogen antigen by immunodiffusion, immunoelectrophoresis (pH 5.2 and 8.6), and crossed immunoelectrophoresis. His unreduced purified fibrinogen had normal migration on polyacrylamide slab gels. Also, the migration in gel slabs of A alpha, B beta and gamma-polypeptide chains, produced by mercaptoethanol reduction of purified patient fibrinogen, was similar to reduced normal fibrinogen. Thrombin-induced total fibrinopeptide release was normal. However, fibrin monomers produced from patient fibrinogen by thrombin (devoid of fibrinopeptides A and B) reaggregated abnormally; fibrin monomers produced by reptilase (devoid of only fibrinopeptides A) reaggregated normally. Fibrin generated from patient plasma in the presence of factor XIII and calcium, was defective in the formation of covalently bonded alpha-alpha polymers and demonstrated an increased susceptibility to the lytic effects of plasmin (generated in vitro by the addition of streptokinase).

Prothrombin Houston: a dysprothrombin identifiable by crossed immunoelectrofocusing and abnormal Echis carinatus venom activation

A 72-yr-old male with a lifelong history of easy bruisability and posttraumatic bleeding had a prolonged prothrombin time and activated partial thromboplastin time. His plasma Stypven, Taipan, and Echis carinatus venom clotting times were prolonged. The presence of a dysprothrombin was confirmed by the discrepancy between plasma prothrombin coagulant activity and prothrombin antigen levels. His plasma prothrombin was capable of being completely absorbed onto and then eluted from barium sulfate. Crossed immunoelectrophoresis of his plasma prothrombin, and normal plasma prothrombin, into agarose containing rabbit anti-human factor II antibody were similar. Crossed immunoelectrofocusing, a procedure combining isoelectric focusing in disc gels with electroimmunoassay in the second dimension, demonstrated that the patient's prothrombin antigen was more basic than normal. The eluate from barium sulfate absorbtion of patient plasma, when reacted with Echis carinatus venom (which directly cleaves prothrombin to thrombin) clotted purified fibrinogen at a rate slower than normal plasma eluate. SDS-slab gel electrophoresis revealed that the prothrombin present in the patient's eluate was cleaved by Echis carinatus venom. These studies suggest that the coagulopathy of prothrombin Houston results from the generation of a dysfunctional thrombin.