Method for isolating mutant cells

A method for isolating a mutant cell that produces a desired compound, comprising:

• (a) providing a starter cell selected from the group consisting of protista, monera, fungi, plant, animal and human cells which:
• (i) is an auxotroph that has a requirement for a first molecule for its growth in a solid growth medium, and mutates spontaneously to lack the requirement at a frequency of less than 10.sup.-10 per cell division,
• the first molecule being selected from the group consisting of sugars, amino acids, growth factors, enzymes, cofactors, cytokines, lymphokines, trophic factors, fatty acids, lipids, interleukins, vitamins, and carbohydrates, the first molecule being other than the desired compound;
• (ii) is resistant to lambdoid bacteriophage infection;
• (iii) excretes a second molecule selected from the group consisting of a vitamin, amino acid, nucleic acid, cofactor, and sugar into the solid growth medium;
• and
• (iv) grows on a solid medium that supports the growth of bacteria;
• (b) modifying the cellular DNA of the starter cell so as to increase the probability that the starter cell will excrete the desired compound;
• (c) providing an Escherichia coli cell which:
• (i) is an auxotroph that requires said second molecule and a polypeptide nutrient for growth on the solid growth medium,
• said second molecule being other than said first molecule or said nutrient, and said nutrient being other than said first molecule;
• (ii) is susceptible to lambdoid bacteriophage infection;
• and
• (iii) excretes the first molecule into the solid growth medium;
• (d) culturing a plurality of modified starter cells and E. coli cells together in the solid growth medium in the presence of a reversibly noninfective modified lambdoid bacteriophage having a head and a tail which:
• (i) has reduced or no capacity for lytic infection;
• (ii) carries a gene encoding said polypeptide nutrient;
• and
• (iii) further comprises a gpV protein having the amino acid sequence set forth in SEQ ID NO:
• 1 linked to an antibody or matrix through a target molecule, wherein the connection between the gpV protein and the target molecule is selected from the group consisting of
• truncation of the carboxy-terminal 24 amino acids of said sequence and genetic fusion of the truncated gpV protein to the target molecule, and
• substitution of an amino acid residue selected from the group consisting of histidine, cysteine, tryptophan, and tyrosine for amino acid position 246 of said sequence and chemical coupling of the target molecule to the substituted gpV protein, and wherein the linkage of the target molecule to the antibody or matrix has rendered the bacteriophage reversibly noninfective,
• the bacteriophage becoming infective if one of the modified starter cells excretes the desired compound,
• whereby the infective bacteriophage infects at least one of the E. coli cells, thereby enabling the infected E. coli cell to produce the nutrient;
• (e) detecting a colony formed when the modified starter cell which excretes the desired compound excretes the first molecule and the infected E. coli cell excretes the second molecule, thereby enabling the E. coli and starter cells to grow;
• and
• (f) isolating the modified starter cell from the E. coli cells in the colony.