of Work: Rates of spontaneous mutation vary greatly among organisms. We have previously described several genomic mutation rates that are characteristic of certain broad groups of organisms. (1) The characteristic mutation rate for DNA-based microbes is about 0.003 per genome replication. The question arises whether this rate also occurs in extremophiles including Archeons. We have characterized the mutation process in the Archeon Sulfolobus acidocaldarius growing at pH 3.5 at 75?C. The measured rate, 0.002, is not different from the characteristic value. The proportion of base-pair substitutions in this organism is lower than in all other DNA-based organisms. (2) The first mutational spectrum for a ribovirus has been obtained using tobacco mosaic virus growing in a host carrying a TMV transgene that complements defects in the 804-base gene that facilitates the spread of virus particles between cells. The most remarakable aspect of the spectrum is the high proportion of mutants carrying multiple mutations, far in excess of predictions based on the mutant frequency. The results suggest that nonheritable, transitory hypermutability occurs from time to time during virus reproduction.
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