ABSTRACT
The conclusive demonstration that gene amplification had occurred in these histidinol-resistant cells was obtained by comparing the intensities of signals from a Southern blot containing serial dilutions of DNA from the resistant and wild-type cells, both electrophoresed in the same agarose gel. A small cDNA library was constructed using mRNA from histidinol-resistant Chinese hamster ovary cells. Histidinol is a competitive inhibitor of histidyl-tRNA synthetase in animal cells. The finding of high levels of mRNA in the resistant mutants then provided the opportunity to develop a strategy for cloning of the homogeneously staining region (HRS) gene. The development of the cell lines with a 30-fold amplification for the HRS locus facilitated the detailed molecular study of HRS. The HRS system has already proved to be a very useful one from the point of view of development of understanding of the structure of a housekeeping gene and in respect to gene amplification.
