Sigma factor is transmitted through egg cytoplasm and its reproduction depends on initial supply and on suitable temperature of 20°C, because it is heat labile at high temperature. It does not need a specific gene and may be found associated with different genotypes. A sensitive fly retains its sensitive trait, even when all its chromosomes are replaced by those of normal fly, suggesting that sigma factor or particle is located in cytoplasm and has properties of non-chromosomal genes or plasma genes. Some physical characteristics of sigma are also known. It seems to be a particle of 0.07 micron in diameter, and contains DNA responsible for its hereditary nature.
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