The expression of the trans-acting transcription factor Spl in mice was defined by a combination of RNA analysis and immunohistochemical localization of the Spl protein. Although ubiquitously expressed, there was an unexpected difference of at least 100-fold in the amount of Spl message in different cell types. Spl protein levels showed corresponding marked differences. Substantial variations in Spl expression were also found in some cell types at different stages of development. Spl levels appeared to be highest in developing hematopoietic cells, fetal cells, and spermatids, suggesting that an elevated Spl level is associated with the differentiation process. These results indicate that Spl has a regulatory function in addition to its general role in the transcription of housekeeping genes.The control of transcription is a key step in the regulation of gene expression. Although specific DNA elements around and within a gene contain the information directing this regulation, proteins that recognize these DNA sequences are the factors actually responsible for modulating RNA polymerase activity. Several sequence-specific DNA-binding transcription factors that are involved in the process of transcriptional regulation have been described, and much has been learned at the molecular level about their structure and function (24,25). Some transcription factors are characterized by cell type-specific expression and are involved in regulating the specialized functions of that cell. For example, the Oct-2 protein is B cell specific (5, 30), and Myo Dl is muscle cell specific (7). In contrast, other factors appear to be ubiquitously expressed and involved in the transcription of genes expressed in all cell types. A well-characterized example of this type of factor is Spl, which is reportedly found in all mammalian cell nuclei (2).Although originally described as a specific factor required for simian virus 40 (SV40) transcription (9, 10), Spl interacts with GC boxes in the promoters of many other genes. For example, cellular genes that contain one or more GC boxes include adenosine deaminase (33), hypoxanthine guanine phosphoribosyl transferase (23) binding sites will require larger amounts of Spl than are required for genes with strong binding sites. Thus, some genes will be more sensitive than others to variations in the level of Spl. Third, the observation that Spl levels increase during SV40 infection (15, 28) implies that Spl levels can be regulated. In this system, alteration of the normal Spl expression may support the SV40 viral life cycle and the accompanying cellular changes.To further investigate the regulation and action of Spl, we have determined Spl expression in different cell types during development of the mouse. In this paper, we show that although Spl is ubiquitously expressed, substantial variations that are likely to play an important regulatory role occur in Spl expression. These studies are relevant to defining (i) the physiological role of Spl in growth and development, (ii) the variations in expressi...

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The complete 184,457-bp sequence of the aromatic catabolic plasmid, pNL1, from Sphingomonas aromaticivorans F199 has been determined. A total of 186 open reading frames (ORFs) are predicted to encode proteins, of which 79 are likely directly associated with catabolism or transport of aromatic compounds. Genes that encode enzymes associated with the degradation of biphenyl, naphthalene,m-xylene, and p-cresol are predicted to be distributed among 15 gene clusters. The unusual coclustering of genes associated with different pathways appears to have evolved in response to similarities in biochemical mechanisms required for the degradation of intermediates in different pathways. A putative efflux pump and several hypothetical membrane-associated proteins were identified and predicted to be involved in the transport of aromatic compounds and/or intermediates in catabolism across the cell wall. Several genes associated with integration and recombination, including two group II intron-associated maturases, were identified in the replication region, suggesting that pNL1 is able to undergo integration and excision events with the chromosome and/or other portions of the plasmid. Conjugative transfer of pNL1 to another Sphingomonas sp. was demonstrated, and genes associated with this function were found in two large clusters. Approximately one-third of the ORFs (59 of them) have no obvious homology to known genes.

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