Observed mutation rates in humans appear higher in male than female gametes and often increase with paternal age. This bias, usually attributed to the accumulation of replication errors or inefficient repair processes, has been difficult to study directly. Here, we describe a sensitive method to quantify substitutions at nucleotide 755 of the fibroblast growth factor receptor 2 (FGFR2) gene in sperm. Although substitution levels increase with age, we show that even high levels originate from infrequent mutational events. We propose that these FGFR2 mutations, although harmful to embryonic development, are paradoxically enriched because they confer a selective advantage to the spermatogonial cells in which they arise.

A full-length cDNA for a membrane transporter was isolated from Brassica napus by its sequence homology to a previously cloned Arabidopsis low affinity nitrate transporter. The cDNA encodes a predicted protein of 589 amino acid residues with 12 putative transmembrane domains. The transporter belongs to a multigene family with members that have been identified in bacteria, fungi, plants, and animals and that are able to transport a range of different nitrogen-containing substrates, including amino acids, peptides, and nitrate. To identify the substrates of this plant gene, we have expressed the protein in Xenopus oocytes. The properties of the transporter are consistent with a proton cotransport mechanism for nitrate, and the voltage dependence of the K m for nitrate was determined. The K m for nitrate was shown to increase from 4 to 14 mM as the membrane voltage became more negative from ؊40 to ؊180 mV. Oocytes expressing the gene could accumulate internal nitrate to concentrations higher than those measured in water-injected controls. A range of different substrate molecules for the transporter was tested, but of these, histidine gave the largest currents, although the affinity was in the millimolar range. The pH dependence of the activity of the transporter was different for the substrates, with histidine transport favored at alkaline and nitrate at acid external pH. Kinetic analysis of the mechanism of histidine transport suggests a cotransport of protons and the neutral form of the amino acid, with the K m for histidine decreasing at more negative membrane voltages. This gene is the first member of this family of transporters for which the transport of two very different types of substrate, nitrate and histidine, has been demonstrated.A family of mammalian peptide transporters (1, 2) has been identified, and sequence comparisons have shown that it includes plant peptide transporters (3-5). The mammalian peptide transporters have been shown to transport a broad range of substrates, including di-and tripeptides (2) and free amino acids (6). The family members are characterized by all having a consensus motif, and they have been named the proton-dependent oligopeptide transporter (POT) family (7), or as most members are peptide transporters, they have also been called the PTR family (8). However, the family also includes plant members that have been identified as nitrate transporters (9, 10). In this paper, we show that another member of this family, isolated from the plant Brassica napus, can transport both the amino acids and nitrate when expressed in Xenopus oocytes.In soil, the nitrate concentrations can vary from Ͼ1 M to Ͼ10 mM depending on factors such as rainfall and fertilizer supply (11). Soil also contains other forms of nitrogen, including ammonium and amino acids, and these may also be nitrogen sources available to plants (e.g. Ref. 12). Nitrate uptake by plants has been shown to have biphasic kinetics, with different affinities for external nitrate; one uptake system has K m values for nitrate in t...

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The nitrate induction of NADH:nitrate reductase mRNA in maize roots, scutella and leaves was investigated in the presence and absence of inhibitors of protein synthesis. In the absence of inhibitors, nitrate treatment caused a fairly rapid (2 to 3 h) increase in the level of the nitrate reductase transcript in all tissues. When cytoplasmic protein synthesis was inhibited by cycloheximide, nitrate reductase mRNA was induced by nitrate in all tissues to levels equal to or greater than those found with nitrate treatment alone. Treatment of maize tissues with cycloheximide in the absence of nitrate had only a small effect on the accumulation of the nitrate reductase mRNA. Inhibition of organellar protein synthesis with chloramphenicol also had little or no effect on nitrate-induced nitrate reductase mRNA accumulation in roots and scutella, but did appear to partially inhibit appearance of transcript in leaves. Excision of scutella in the absence of nitrate was sufficient to cause some accumulation of the nitrate reductase transcript. Since cytoplasmic protein synthesis was not required for expression of nitrate reductase transcripts, induction of these transcripts by nitrate is a primary response of maize to this environmental signal. Thus, it appears that the signal transduction system mediating this response is constitutively expressed in roots, scutella and leaves of maize.