It had previously been observed that DNA-directed cell-free synthesis of the galactose enzymes of
E. coli differs in one striking aspect from the
in vivo synthesis: whereas
in vivo the expression of genes located distal to a polar mutation is severely reduced, these genes are fully expressed
in vitro.
This cell-free system has been used to develop conditions for restoration of polar effects in vitro. It is shown that the addition of certain protein fractions from E. coli extracts to the cell-free enzyme synthesis permits the expression of polar effects of insertion mutations as well as of an amber mutation. This polarity-restoring activity is not present in extracts from E. coli cells carrying a mutation in gene suA.
Restoration of polarity has also been demonstrated in an RNA-directed cell-free system for the synthesis of the galactose enzymes. This observation excludes any model implying termination of transcription as sole cause of polar effects exerted by insertions as well as nonsense mutations.
Communicated by H. G. Wittmann