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If genes have been assembled from exon subunits, the frequency with which exons are reused leads to an estimate of the size of the underlying exon universe. An exon database was constructed from available protein sequences, and homologous exons were identified on the basis of amino acid identity; statistically significant matches were determined by Monte Carlo methods. It is estimated that only 1000 to 7000 exons were needed to construct all proteins.

DNA polymorphism in the Y chromosome, examined at a 729-base pair intron located immediately upstream of the ZFY zinc-finger exon, revealed no sequence variation in a worldwide sample of 38 human males. This finding cannot be explained by global constraint on the intron sequence, because interspecific comparisons with other nonhuman primates revealed phylogenetically informative sequence changes. The invariance likely results from either a recent selective sweep, a recent origin for modern Homo sapiens, recurrent male population bottlenecks, or historically small effective male population sizes. A coalescence model predicts an expected time to a most recent common ancestral male lineage of 270,000 years (95 percent confidence limits: 0 to 800,000 years).

Bacterial RNase P is an essential ribonucleoprotein composed of a catalytic RNA component (encoded by the rnpB gene) and an associated protein moiety (encoded by rnpA). We construct a system that allows for the deletion of the essential endogenous rnpA copy and for its simultaneous replacement by a heterologous version of the gene. Using growth rate as a proxy, we explore the effects on fitness of heterologous replacement by increasingly divergent versions of the RNase P protein. All of the heterologs tested complement the loss of the endogenous rnpA gene, suggesting that all existing bacterial versions of the rnpA sequence retain the elements required for functional interaction with the RNase P RNA. All replacements, however, exact a cost on organismal fitness, and particularly on the rate of growth acceleration, defined as the time required to reach maximal growth rate. Our data suggest that the similarity of the heterolog to the endogenous version — whether defined at the sequence, structure or codon usage level — does not predict the fitness costs of the replacement. The common assumption that sequence similarity predicts functional similarity requires experimental confirmation and may prove to be an oversimplification.

Last year, some 50 million pounds of antibiotics were used in the United States, an amount that would correspond to roughly 5 ta blespoons?or 75 doses?of antibiotics per person. In fact, much of this antibi otic?as much as 70 percent by some estimates?is being used not to treat infections, but instead to promote food production, as antibiotics have become a key ingredient in the American food chain. We are, in short, marinating the living world in antibiotics. Against this backdrop, the emergence of antibiotic-resistant pathogenic bacte ria can hardly come as a surprise. Anti biotics, after all, are used to suppress and kill bacteria, and bacteria, like every other living thing, have no greater evo lutionary imperative than to stay alive. Our overuse of antibiotics methodi cally rewards any bacterium fortunate enough to carry a mutation that confers even slight resistance to the substance in its environment. The less-fortunate

Lederman then said, "I don't know if we have such a thing as a prairie hat," provoking more laughter. His subse quent speech encouraged his people to keep physics going at Fermilab for another 10 to 15 years. And indeed, Fermilab's program continued to flour ish after Lederman stepped down the following year, and also beyond the 1993 decision of Congress to terminate the Superconducting Super Collider. The Main Injector, an upgrade of the Main Ring accelerator in the early 1990s, led to Fermilab's discovery in 1995 of the top quark, the last of the six quarks. The book draws to a close at that point. But Fermilab's story was not over then; it continues today. Since the discovery of the top quark, the laboratory has continued to pursue a rich program of research in physics. However, the Tevatron will shut down within the next few years. Internation al attention has shifted to the Large Hadron Collider in Europe. Fermilab's long-term future is cur rently unclear, although research con tinues there on questions of neutrino mass, the Higgs boson, supersymmetry and new properties of matter. Ambi tious plans are in the works for new high-intensity beams, for astrophysical initiatives, and for perhaps being the eventual site of the International Linear

We report a rapid technique, based on the polymerase chain reaction (PCR), for the direct targeting, enhancement, and sequencing of previously uncharacterized cDNAs. This method is not limited to previously sequenced transcripts, since it requires only two adjacent or partially overlapping specific primers from only one side of the region to be amplified. These primers can be located anywhere within the message. The specific primers are used in conjunction with nonspecific primers targeted either to the poly(A)+ region of the message or to an enzymatically synthesized d(A) tail. Pairwise combinations of specific and general primers allow for the amplification of regions both 3' and 5' to the point of entry into the message. The amplified PCR products can be cloned, sequenced directly by genomic sequencing, or labeled for sequencing by amplifying with a radioactive primer. We illustrate the power of this approach by deriving the cDNA sequences for the skeletal muscle alpha-tropomyosins of European common frog (Rana temporaria) and zebrafish (Brachydanio rerio) using only 300 ng of a total poly(A)+ preparation. In these examples, we gained initial entry into the tropomyosin messages by using heterologous primers (to conserved regions) derived from the rat skeletal muscle alpha-tropomyosin sequence. The frog and zebrafish sequences are used in an analysis of tropomyosin evolution across the vertebrate phylogenetic spectrum. The results underscore the conservative nature of the tropomyosin molecule and support the notion of a constrained heptapeptide unit as the fundamental structural motif of tropomyosin.