Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Biochemical Genetics and the Synthesis of Nucleic Acids and Proteins
Genetic Methods
Cistron

How can we answer the question of whether Mutations are localized within the same Gene, in closely linked genes, or in genes located at some distance from one another? This question can be addressed using a complementation test. If two mutant Bacteriophages carry mutations in different genes, simultaneous infection of a bacterium with both mutant phages often results in successful phage reproduction within the host. Because each phage possesses an intact gene for one of the two Proteins affected, all required genetic Functions are supplied. Conversely, if both mutant phages have defects in the same gene, they will fail to Complement each other's functions upon co-infection. Such an experiment is frequently referred to as a cis-trans comparison. Simultaneous infection with two distinct mutants constitutes the trans-test. Meanwhile, the cis-test serves as a control: the bacterium is infected simultaneously with a recombinant carrying both mutations on the same DNA molecule and a standard phage. In this case, Replication should proceed normally.

When phages carrying various mutations in the rII region were analyzed using the cis-trans test, it became clear that two distinct genes exist—rIIA and rIIB—which were identified solely through their complementation behavior in the cis-trans test. To designate a segment of DNA identified as a genetic unit, Benzer proposed the term cistron. For most purposes, the terms gene and cistron are synonymous, and both are used rather interchangeably in the biochemical literature. From a strict genetic standpoint, however, the term cistron is more precise.

At the time the mapping of the rII region was carried out, little was known about the Functions of the proteins encoded by these two cistrons. Today, we know that both the rIIA and rIIB proteins are integrated into the membranes of bacterial Cells infected with the phage [132, 133]. There, they facilitate the lysis of infected cells, causing the plaques formed by rII mutants to be larger in size and have sharper edges than those of standard plaques.