Polarity Effects in the Hisg Gene of Salmonella Require a Site within the Coding Sequence

A single site in the middle of the coding sequence of the hisG gene of Salmonella is required for most of the polar effect of mutations in this gene. Nonsense and insertion mutations mapping upstream of this point in the hisG gene all have strong polar effects on expression of downstream genes in the operon; mutations mapping promotor distal to this site have little or no polar effect. Two previously known hisG mutations, mapping in the region of the polarity site, abolish the polarity effect of insertion mutations mapping upstream of this region. New polarity site mutations have been selected which have lost the polar effect of upstream nonsense mutations. All mutations abolishing the function of the site are small deletions; three are identical, 28-bp deletions which have arisen independently. A fourth mutation is a deletion of 16 base pairs internal to the larger deletion. Several point mutations within this 16-bp region have no effect on the function of the polarity site. We believe that a small number of polarity sites of this type are responsible for polarity in all genes. The site in the hisG gene is more easily detected than most because it appears to be the only such site in the hisG gene and because it maps in the center of the coding sequence.     

Expression of the cloned uvrB gene of Escherichia coli: dependency on nonsense suppressors.

Recombinant plasmid pNP5, consisting of plasmid pMB9 on which the uvrB gene is cloned, fully complements for the defects due to chromosomal uvrB mutations in the presence of the amber suppressor sup-6 or supF. Correndonuclease II activity was also completely restored in in UvrB strains containing both plasmid pNP5 and amber suppressor sup-6, as compared with the parental UvrB+ strain. It is shown that the amber mutation which interferes with the expression of the cloned uvrB gene is located outside this gene. Apparently, the amber mutation exerts a polar effect on uvrB expression that is relieved by sup-6 or supF. Introduction of a rho mutation into suppressor-free UvrB strains, harboring pNP5, did not relieve the polarity caused by the amber mutation.     

Preferential induction of AT-TA transversion, but not deletions, by chlorambucil at the hisG428 site of Salmonella typhimurium TA102.

The chemotherapeutic agent chlorambucil effectively induces deletion mutations in mouse germ cells. The possibility that this chemical also effectively induces deletion mutations in bacterial DNA was examined using Ames Salmonella tester strains. Chlorambucil was mutagenic only to strains TA102 (hisG428, rfa/pKM101) and YG2975 (hisG46, rfa/pKM101) when S9 mix was absent. Since strain TA102 can detect short deletions, the mutational changes of TA102 induced by this agent without S9 mix were directly determined by the DNA sequencing technique. It turned out that chlorambucil did not induce deletion mutations but preferentially induced AT-TA transversions at the hisG428 site of plasmid pAQ1 of strain TA102. These results caution that the positive results induced by chlorambucil in mutagenicity tests do not necessarily mean the occurrence of deletions.