The h region of bacteriophage T2H, with special reference to an anomalous mutant.

I N bacteriophage T2 the host range ( h ) region controls the specific adsorption properties of the phage particles. The original isolate, T2h+, adsorbs to E. coli strain B, but not to the mutant strain B/2. Spontaneous T2h mutants can be isolated from T2hf which now adsorb to both cell strains. STREISINGER and FRANKLIN, working with T2L, first investigated the genetic properties of this host range region 1956). They could detect no h+ recombinants in crosses between different h isolates; however, crosses between different h+ isolates derived from an h mutant usually yielded a small number of recombinants. From the mutational point of view the “wild type” of the host range region is thus its h configuration. This determines the formation of tail fibers which adsorb both to B and B/2 bacteria ( FRAXKLIN 1961). One particular class of alteration to this configuration leads to the formation of h+ fibers, which adsorb now to B. but not to B/2 bacteria. The results reported in the first part of this paper establish the same relationship between the h and h+ mutants in T2H as exists between those mutants in T2L. Eleven independently isolated h+ mutants were found to be closely linked. and to map at five distinguishable sites. These sites were then ordered in threefactor crosses which employed the outside marker r13 (rapid lysis. plaque morphology mutant), which in T2H lies close to the h region. During these studies one of the h+ isolates ( h + l l ) showed aberrant properties. behaving in some respects as if it were a double mutant. An intensive genetic analysis was therefore made of h+ 11 ; its mutational pattern was compared to the patterns of the other mutants before and after treatment with several mutagens. To explain the experimental results we were led to consider the possibility that the mutant was formed by an acridine-type mutational event; that is. by the addition or deletion of a base pair in the h region. Although we have not been able to confirm this idea, we reconsidered the types of recombinational event which could be expected to occur in crosses with mutants of the acridine type. The investigation of the properties of this atypical mutant, with a discussion of their possible implications, is the basis for the second part of this paper.