Molecular Genetics

Facts

The fd bacteriophage contains a single-stranded DNA genome of 6 knt. Strains of fd can be distinguished based on the E.coli strain from which they were isolated: fd K from E.coli K and fd B from E.coli B. The results of plaque assays of fd K on each E.coli K and E. coli B are diagrammed (ref). Phage resulting from fd K infection of E.coli B were retested and gave many plaques on both strains. Analysis of base composition revealed that each phage DNA contained 6-methylaminopurines. Amounts of 6-methylaminopurine varied with phage source (see table below).

		6-meA/ 6 knt
	fd K	2
	fd B	4

Interpretations

• Phage fd K is "restricted" in its growth on E.coli B. Phage fd K is " modified" during growth on E.coli B.
• Modification consists of the methylation of a small number of specific DNA bases.
• Restriction is a mechanism used by bacteria to protect their genomes against invasion by DNA from unrelated organisms.

Further information

• The phenomena of restriction and modification led to the discovery of restriction endonucleases by Arber, Smith and Nathans (1978 Nobel prize).
• Restriction results from endonucleolytic cleavage of the DNA.
• Restriction endonucleases have become very useful in molecular genetic engineering.

This is page 331 of Molecular Genetics by Ulrich Melcher, © 1997, 1998