THE ORIGIN OF ADAPTIVE MUTATIONS - EXPLAINING THE MUTATIONAL SPECTRA OF LAC(-COLI STRAIN FC40() REVERTANTS OF THE ESCHERICHIA)

Reversion of lacI33-lacZ frameshift mutation to produce Lac(+) colonie s takes place by demonstrably different mechanisms in growing and star ving cultures of strains (such as FC40) harbouring this mutation. The revertants appear to arise in starving cells only under conditions whe re they can immediately promote growth. This is reminescent of Lamarck ism. Here we propose a mutagenic mechanism which is Darwinian inasmuch as it is blind to the adaptive fitness of the mutants but can explain the mutational spectra seen in FC40 and certain other strains. This m echanism can produce mutations only in the neighbourhood of a methylat able cytosine.