Protein synthetic error frequency, determined in cell-free extracts as delta leu/delta phe incorporation following poly(U) stimulation, has been found to decrease progressively in several strains of human diploid fibroblasts during their limited replicative lifespan. To explore the basis of this phenomenon, we followed a mass (uncloned) culture of one normal strain at 13 stages of its replicative lifespan. We found a progressive tenfold decline in error frequency that was inversely correlated with passage level (r = -.93, p less than .001). This could not be ascribed to the slow rates of replication associated with fibroblast senescence because slowing of growth by serum deprivation did not change error frequency. Additionally, terminal mass cultures maintained for 16 wk at saturation density to minimize cell selection did not change error frequency over this time. Error frequencies in 12 individual clones purified from the parental culture did not decline on repeated passage, either remaining constant or, in two clones, rising abruptly three- to five-fold after initial assays. Error frequencies of clones showed a weak inverse correlation with growth vigor but not with the maximum doubling number. We conclude that selective pressures favor more vigorously dividing clones with low protein synthetic error frequencies leading to their predominance in mass cultures.

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