Multiplicity of genome equivalents in the radiation-resistant bacterium Micrococcus radiodurans

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Abstract
The complexity of the genome of M. radiodurans was (2.0 .+-. 0.3) .times. 109 daltons by DNA renaturation kinetics. The number of genome equivalents of DNA per cell was calculated from the complexity and the content of DNA. A lower limit of 4 genome equivalents/cell was approached with decreasing growth rate. Thus, no haploid stage appeared to be realized in this organism. The replication time was estimated from the kinetics and amount of residual DNA synthesis after inhibiting initiation of new rounds of replication. From this, the redundancy of terminal genetic markers was calculated to vary with growth rate from 4 to approximately 8 copies/cell. All genetic material, including the least abundant, is thus multiply represented in each cell. The potential significance of the maintenance in each cell of multiple gene copies is discussed in relation to the extreme radiation resistance of M. radiodurans.