Reversions of two proline-requiring auxotrophs of Haemophilus influenzae by n-methyl-n'-nitro-n-nitrosoguanidine and hydrazine

Abstract

New mutation detection systems are described for Haemophilus influenzae. They involve two independently isolated proline auxotrophs which appear to be mutants at different sites in a proline locus (proB) that is very closely linked to a locus (thd) for thymidine requirement. One of the mutants, proB1, appears to revert to prototrophy only by mutations at the locus. The other, proB2, reverts both by mutation at the locus and by unlinked suppressors. The latter account for about 90% of the reversions induced by MNNG and by HZ. The close linkage of proB to thd was used to distinguish between true revertants and suppressors by a transformation test. A comparison was made between the mutation induction kinetics of the different classes of revertants and mutations to novobiocin resistance with MNNG and HZ. The very different induction kinetics for these two mutagens previously reported for the novobiocin resistance system were also found for the proline systems. There were some differences between the detection systems, however, in the frequency of induced mutation relative to the spontaneous frequency and, in one case, in the form of the induction curve. It is concluded that the major features of the induction curves reflect the amount of damage done to DNA and so are general for all systems, but that there are some features which are locus-or site-specific.