The transformation of both recipient strands of pneumococcal DNA.

Abstract

SUMMARY: It has been established that, in pneumococcus, donor sites on transforming deoxyribonucleic acid (DNA) can be divided into at least two types, known as low-efficiency (LE) sites and high-efficiency (HE) sites, by comparing the number of transformants of a particular character with that obtained for a standard reference gene ( Ephrussi- Taylor, Sicard & Kamen, 1965 ; Lacks, 1966 ). The reference gene used by Ephrussi- Taylor and her colleagues was str-41, whilst Lacks used the sulf-d gene. Ephrussi- Taylor & Gray (1966) proposed that LE markers are integrated into both chains of the recipient DNA by an excision and repair mechanism, whilst HE markers have to wait for a further division before both the recipient strands become altered. Evidence that the LE markers are transmitted into both strands whilst HE markers are transmitted to only one strand has been put forward by Ephrussi-Taylor (1966) , Gray & Ephrussi-Taylor (1967) and Louarn & Sicard (1968) . Louarn & Sicard (1969) have taken advantage of this difference between the integration of the markers to show that either of the recipient strands of DNA can be transformed by the HE marker. They calculated the ratio R = (D.N)/(A.B), where D = number of doubles transformed for genes A and B, N = the total colony-forming units (c.f.u.) present, and A and B the number of transformants for genes A and B respectively. By comparing the ratios obtained for pairs of markers having the composition HE-HE, LE-LE, or HE-LE, Louarn and Sicard showed that the results were consistent with either strand being transformed by an HE marker. They also discussed the effect of replication events on the value of R, particularly with respect to the location on the chromosome of such events in relation to the markers. The results reported below support these conclusions.