Evidence favoring a frame shift mechanism for ICR-170 induced mutations in Drosophila melanogaster.

MONOFUNCTIONAL quinacrine mustard, 2-methoxy-6-chloro-9(3[ethyl2-chloroethyl] aminopropylamino) acridine dihydrochloride, ICR-170*, consists of two portions: an acridine ring and an alkylating mustard. The complex chemical structure of PCR-170 suggests that mutations could be produced by several different mechanisms. If the nitrogen mustard component of the molecule is the effective part of the molecule, ICR-170 should act simply by radiomimetic alkylation. Alkylating mustards have long been known to be mutagenic in Drosophila (AUERBACH 1945). It is also possible that the quinacrine ring is the mutagenic component and that ICR-170 acts as an acridine. The work of ORGEL and BRENNER (1961 ) has shown that many acridines are mutagenic in phage T4. A third possibility for ICR-170 would be a complex one, in which both components are directly involved in the mutational process. Studies of phage T4 have supported the theory of BRENNER, BARNETT, CRICK and ORGEL (1961) which argued that acridine type mutants were due to the addition or deletion of one or more base pairs. Such mutations would be caused by shifts of the reading frame in the DNA (CRICK, BARNETT, BRENNER and WATTS-TOBIN 1961 ; STREISINGER et al. 1966). These acridine type “frame shift” mutations result in changes of a long sequence of adjacent amino acids which constitute a major portion of a protein. The acridine type mutations are characterized also by the complete loss of protein function, by noncomplementation, and by reversion specificity ( ORGEL and BRENNER 1961 ) . Alkylating agents differ from the acridines (BAUTZ and FREESE 1960) ; they readily induce mutations of the base substitution or “missense” type. This mutational specificity has been critically determined by KRIEG (1963) for ethyl methanesulfonate and by LOEBBECKE (1963) for nitrogen mustard. Agents which produce base substitution types of mutants readily produce temperature sensitive mutations in many different structural genes (EDGAR, DENHART and EPSTEIN 1964). EDGAR (personal communication) has shown that temperature sensitive mutants are not induced by acridines in phage T4. In microbial systems, particularly phage T4, it is possible to distinguish readily these alternative mechanisms proposed for ICR-170. Experiments in T4 phage support the conclusion that ICR-170 is mutagenic and acts as an acridine agent (SEDEROFF 1966). AMES and WHITFIELD (1966) have used ICR-170 on Sulmo-