Protein-tyrosine kinases are implicated in the control of normal and neoplastic cell growth. We have used molecular cloning strategies to characterize a lymphocyte-specific protein-tyrosine kinase gene distinct from but closely related to src and yes. This gene, encoded by a genetic locus defined here as lskT, is rearranged and overexpressed in the murine T cell lymphoma LSTRA. Thus alterations in the structure or expression of this protein-tyrosine kinase gene may in some cases mediate neoplastic transformation. In addition, transcription of the normal lskT gene is restricted to cells of lymphoid origin. We infer that the lskT-encoded protein-tyrosine kinase may aid in transducing proliferative or differentiative signals unique to lymphocytes.

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Loss of the ability of Pseudomonas syringae pv. "phaseolicola" NPS3121 to elicit a hypersensitive response on tobacco and other nonhost plants was associated with loss of pathogenicity on the susceptible host bean. Eight independent, prototrophic transposon Tn5 insertion mutants which had lost the ability to elicit a hypersensitive response on tobacco plants were identified. Six of these mutants no longer produced disease lesions on primary leaves of the susceptible bean cultivar Red Kidney and failed to elicit a hypersensitive response on the resistant bean cultivar Red Mexican and on the nonhost plants tomato, cowpea, and soybean.The two remaining mutants had reduced pathogenicity on Red Kidney bean and elicited variable hypersensitive responses on the other plants tested. Southern blot analysis indicated that each mutant carried a single independent Tn5 insertion in one of three EcoRI fragments of about 17, 7, and 5 kilobases. Marker exchange mutagenesis further supported the conclusion that the pleiotropic mutant phenotype was not associated with multiple TnS insertions. A genomic library of the wild-type strain was constructed in the cosmid vector pLAFR3. A recombinant plasmid, designated pPL6, that carried P. syringae pv. "phaseolicola" genomic sequences was identified by colony hybridization. This plasmid restored the wild-type phenotype to all but one mutant, suggesting that genes affected by the insertions were clustered. Structural analysis of pPL6 and the wild-type genome indicated that the 17-and 5-kilobase EcoRI fragments were contiguous in the strain NPS3121 genome.