The retroviral oncogene x-myb encodes a nuclear, sequence-specific DNA-binding protein. To investigate the possibility that \-myb encodes a transcriptional regulator, we used a transient cotransfection assay to explore the potential of \-myb to influence the expression of other genes. We found that expression of a chicken lysozyme promoter/CAT gene construct was activated by \-myb in the presence of myft-specific binding sites. Activation was not observed with a truncated \-myb protein lacking its DNA-binding domain. We also observed that expression of a hybrid human HSP70 promoter/CAT gene, lacking myb-specific binding sites, was activated by \-myb. However, in this case, the truncated \-myb protein, which lacked its DNA-binding domain, also activated HSP70/CAT expression, indicating that trdns-activation of this gene construct was independent of the sequence-specific DNA-binding activity of the \-myb protein. These observations suggest that \-myb encodes a trans-activator and that activation of gene expression occurs by two different mechanisms, one of which involves specific binding of v-myb protein to the regulated gene.

The transforming protein encoded by the v-myb oncogene is a sequence-specific DNA-binding protein that is thought to be involved in the regulation of gene expression. The N-terminal region of the v-myb protein is composed of two highly conserved tandem repeat sequences of unknown function. It has been speculated that the N-terminal v-myb repeats might be crucial for DNA-binding, since N-terminal deletions destroy the DNA-binding activity of the v-myb protein. Here, we have studied the v-myb DNA-binding domain in more detail. Our results show that the N-terminal region of the v-myb protein is sufficient for specific DNA-binding. Dissection of this region suggests that both repeats are required for DNA-binding, but that both repeats play different roles in v-myb protein DNA interaction. We also show that the myb repeats of a drosophila melanogaster homolog of c-myb function as sequence-specific DNA-binding domain. Our results support the view that specific sequence-recognition, mediated by the conserved myb repeats, is a general feature of myb-related proteins.

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ABSTRACT-In order to elucidate sublethal cytopathological alterations in hepatocytes, mature male rainbow trout Oncorhynchus mykiss were exposed to 1, 10. 50, and 100 ng I-' technical grade endosulfan (ThiodanTM, 70% a-and 30 % P-isomers) for 28 d. Whereas stereological parameters, i.e. relative volumes and numbers of cell organelles, were unaffected, qualitative ultrastructural alterations were detectable from 10 ng I-' endosulfan. The No-Observed-Effect Concentration (NOEC) for cytological alterations was determined to be 1 ng I-' endosulfan, i e 3 orders of magnitude below the LC,, value Cytological effects that were probably of an adaptive nature included proliferation of SER and circular arrays of RER indicating induction of mixed-function oxygenases (MFO) as well as an increase in lysosomal elements at 50 and 100 ng I-' endosulfan, probably due to enhanced cellular turnover In addition, at 250 ng 1-' endosulfan, degenerative effects such as dilation of intermembranous spaces in mitochondria, deformation of mitochondria, myelin formation in peroxisomes and cytoplasm, and vesiculation and dilation of RER cisternae were observed. Although there was no indication of specific sublethal modes of toxic action except for MFO induction, the present study indicates that endosulfan has toxic impacts at concentrations of environmental relevance.

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