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Study co-author Gregory M. Cochran says: “History looks more and more like a science fiction novel in which mutants repeatedly arose and displaced normal humans – sometimes quietly, by surviving starvation and disease better, sometimes as a conquering horde. And we are those mutants.”
It is sometimes claimed that the pace of human evolution should have slowed as cultural adaptation supplanted genetic adaptation. The high empirical number of recent adaptive variants would seem sufficient to refute this claim.
But there is another angle too: exogenous or extrinsic factors, natural selection as we conventionally understand it. These exterior parameters frame the selection coefficients which drive mutations toward extinction or sweep them up to fixation. And, importantly, some of these parameters are strongly shaped by population size as well. Consider disease, which is likely strongly subject to density and interconnectedness. A world with more humans is a world with more hosts, and remember that many of the arguments for increased rates of substitution derive from pathogen models whose effective population size is contingent upon warm bodies available to infect!
Dutch women who gave birth during the winter of 1944 (known as the famine winter in Holland) had smaller than average babies. That's no surprise - but those babies grew up to have kids of their own, who were also smaller than average. I think there's some evidence for the effect in the next generation as well.
Structural, biochemical, and genetic techniques were applied to investigate the function of FtsJ, a recently identified heat shock protein. FtsJ is well conserved, from bacteria to humans. The 1.5 Å crystal structure of FtsJ in complex with its cofactor S-adenosylmethionine revealed that FtsJ has a methyltransferase fold. The molecular surface of FtsJ exposes a putative nucleic acid binding groove composed of highly conserved, positively charged residues. Substrate analysis showed that FtsJ methylates 23S rRNA within 50S ribosomal subunits in vitro and in vivo. Null mutations in ftsJ show a dramatically altered ribosome profile, a severe growth disadvantage, and a temperature-sensitive phenotype. Our results reveal an unexpected link between the heat shock response and RNA metabolism.
Beyond the Chaperone/Protease ParadigmThe majority of heat shock proteins characterized function either as molecular chaperones or as proteases (). However, the recent discovery and functional analysis of several E. coli heat shock proteins has revealed functions that significantly alter this paradigm. We have shown that FtsJ, a well-conserved heat shock protein, is structurally related to methyltransferases and is involved in 23S rRNA methylation in the cell. We have previously described another RNA binding heat shock protein, Hsp15, that binds with high affinity to 50S ribosomal subunits (). Hsp15's crystal structure revealed that it comprises a novel RNA binding motif shared by over 500 sequenced proteins including ribosomal protein S4 and some tRNA synthetases ( ). Functional analysis suggested that Hsp15 is involved in recycling 50S ribosomal subunits that are blocked by nascent chains. An additional protein involved in RNA metabolism, the tRNA dimethylallyl diphosphate transferase MiaA, is under heat shock regulation ( ). The described functions of all of these novel heat shock proteins suggest that protein damage control is clearly not the sole role of the heat shock proteins in the cell. Our results show that RNA-related functions are also important in the heat shock response.