An unusual pattern of spontaneous mutations recovered in the halophilic archaeon Haloferax volcanii

Spontaneous mutations in the orotate:phosphoribosyl transferase (pyrE2) gene of the halophilic archaeon Haloferax volcanii were selected by 5-fluoroorotic acid plus uracil at a rate of similar to 2 X 10(-8)/cell division in fluctuation and null-fraction tests but similar to 6 X 10(-8)/cell division in mutation-accumulation tests. The corresponding genomic mutation rates were substantially lower than those observed for other mesophilic microbial DNA genomes on the basis of similar target genes. The mutational spectrum was dominated by indels adding or deleting Multiples of 3 bp. Properties of the organism contributing to this unusual mutational pattern may include phenotypic lag caused by a high chromosomal copy number and efficient promotion of strand misalignments by short direct repeats.