Galactose-specific messenger ribonucleic acid contents in Escherichia coli

A method is described for measuring the proportion of galactose-specific mRNA (gal-mRNA) in the total RNA extracted from pulse-labelled cells of Escherichia coli K12, by DNA-RNA hybridization with DNA prepared from bacteriophage lambdadg. RNA from wild-type E. coli was compared with RNA from a homogenote carrying the gal operon both in the chromosome and in a substituted sex-factor, and with RNA from a deletion strain that carried the galactose operon only in the exogenote. In each case the cultures were induced with fucose. Under these conditions the amount of gal-mRNA was found to be proportional to the content of galactokinase in the different cultures, and to the gene frequency. The amounts of gal-mRNA in an O(c) mutant and an R(-) mutant were also proportional to the observed contents of galactokinase. In cultures repressed for the enzymes of the galactose operon with thiomethylgalactoside, the content of gal-mRNA was higher than expected from the content of galactokinase. Possible explanations of this finding are discussed.