Although dissolved organic nitrogen (DON) is beginning to be seen as a potentially important nitrogen source for phytoplankton, much remains to be learned about its components and their utilization. Emiliania huxleyi, a cosmopolitan eukaryotic phytoplankton species abundant in oligotrophic oceans and during blooms in some coastal regions, was screened for use of various DON compounds. Hypoxanthine and other purines support the nickeldependent growth of most E. huxleyi strains. Acetamide and formamide but not longer chain aliphatic amides were found to be excellent nitrogen sources for growth; other phytoplankton were also found to utilize acetamide but not formamide. In E. huxleyi, small amides are transported into the cell followed by degradation to ammonia, possibly by amide-specific enzymes. The related molecules hydroxyurea and thiourea were toxic to the cells and caused an increase in fluorescence consistent with blockage of photosystern II. This fluorescence increase was inhibited by urea and acetamide, suggesting transport of hydroxyurea, thiourea, urea, and acetamide by the same or closely related transporters.Dissolved organic nitrogen (DON) is the major chemical form of N, other than N gas, in marine waters. This pool of N includes a range of compounds whose concentrations have been measured (e.g. amino acids and urea), but is mostly composed of compounds whose identities and concentrations are poorly known (Sharp 1983;Antia et al. 1991;Hopkinson et al. 1993). Because N is thought to control primary productivity in some areas of the world's oceans, the production rates and biological availability of this pool are clearly important issues. 15N tracer experiments suggest that this pool can be rapidly produced from inorganic N additions and can be available for subsequent utilization (Bronk and Glibert 1991Bronk et al. 1994).The study of the biological utilization of DON can take several approaches. One method is the generation and use of lSN-labeled natural DON for uptake experiments (Bronk and Glibert 1993).A more reductionist approach is the characterization of the turnover times (flux and concentration) of individual known compounds added to natural samples. This second approach is limited by the range of compounds that can be chemically analyzed in seawater or are available as labeled tracers. Part of this approach is also the characterization of the metabolic capabilities of microorganisms. It has been known for some time, for example, that photosynthetic microorganisms do not rely solely on inorganic N sources (a paradigm coming out of an agricultural model; Mills 1989), but are also able to utilize organic forms of nitrogen (reviewed in Antia et al. 1991). However, a large range of organisms and a range of nitrogenous compounds and N moieties remain to be examined as sources of N for phytoplankton and other marine microbes. Just as marine AcknowledgmentsWe thank John Koke for help with media preparation and some biochemical assays and Jackie Collier, Sonya Dyhrman, and two anonymous review...

No abstract

No abstract

Cis-regions and trans-factors controlling TCL1 oncogene expression are not known. We identified the functional TCL1 promoter by mapping four transcriptional start sites 24 -30 bp downstream of a TATA box. A 424-bp fragment upstream of the major start site showed robust promoter activity comparable with SV40 in both TCL1 expressing and non-expressing cell lines. Additional constructs spanning 10 kb upstream and 20 kb downstream of the start site showed only modest increases in reporter activity indicating that TCL1 expression is primarily controlled by the promoter. Ten putative Sp1-binding sites were identified within 300 bp of the start site, and three of these specifically bound Sp1. A dosedependent transactivation of the TCL1 promoter with Sp1 addition in Sp1-negative Drosophila SL2 cells was observed, and mutation of the three identified Sp1-binding sites significantly repressed reporter gene expression in 293T cells, confirming a key role for Sp1 in activating the TCL1 promoter in vivo. In TCL1 silent cell lines, CpG DNA methylation was rarely observed at functional Sp1 sites, and methylation of a previously reported NotI restriction site was associated with dense CpG methylation rather than endogenous TCL1 gene silencing. Together, these results indicate that Sp1 mediates transactivation of the TCL1 core promoter and that TCL1 gene silencing is not dependent on mechanisms involving Sp1 and NotI site methylation.