Passage IV

To grow on a medium (a nutrient system) that lacks arginine (an amino acid), the bacterium E. coli must synthesize arginine from the medium. Figure 1 shows a portion of the reaction pathway for the synthesis of arginine in E. coli. Each of these reactions is catalyzed by an enzyme (E1-E4). In the first reaction, acetylornithine is the precursor, ornithine is the product, and E1 is the enzyme. In the second reaction, ornithine is the precursor, citrulline is the product, and E2 is the enzyme.

Table 1 lists the E. coli genes that normally code for the enzymes in Figure 1.

Sometimes a gene that normally codes for an enzyme is damaged in such a way that the enzyme is not produced. The pathway then shuts down at the reaction catalyzed by that enzyme. As a result, the precursor increases in concentration and the product is not produced. An undamaged gene is labeled with plus sign (for example, arg1⁺). A damaged gene that cannot code for its enzyme is labeled with a minus sign (for example, arg1^-).

Experiment

A biologist grew wild-type (naturally occurring) E. coli on minimal medium (MM), a medium that lacks arginine.

To induce genetic damage, the biologist exposed wild-type E. coli to radiation. She then identified those E. coli that could no longer synthesize arginine from MM. She tested these E. coli on various media, classifying them into 5 types depending on the media on which they grew (see Table 2).

In Table 2, an “x" indicates that a given type could grow on given medium and thus could synthesize arginine from that medium.

Table 2 adapted from Anthony J. F. Griffiths et al, Genetic Analysis, 5th ed. ©1993 by W. H. Freeman and Company.