Relation Between the Transforming Activity of a Marker and Its Proximity to the End of the DNA Particle

Transforming pneumococcal DNA is inactivated by treatment with restriction enzymes. For mutations belonging to the same locus (amiA locus), the extent of inactivation depends strongly upon the mutations and the enzymes. Two EcoRI and one BamHI restriction sites have been located within the amiA locus. After treatment of donor DNA with either one of these enzymes, the lowest transforming activity is observed for mutations that map near restriction sites. This effect of proximity to the nearest end of the DNA fragment extends over a distance of 1,400 nucleotides. The curve of transforming activity versus DNA size obtained with endonuclease-generated DNA fragments is very similar to that obtained previously with mechanically sheared DNA. Both curves show a striking slope change for donor DNA size around 2,700 base pairs, i.e. twice the length found for the extent of the 'end effect'. We suggest that for donor DNA fragments larger than 2,700 base pairs the transforming activity depends mainly upon the size of donor whereas for donor DNA fragments shorter than 2,700 base pairs both a size-dependent phenomenon and the 'end effect' contribute to reduce drastically the transforming activity.