INTEGRATION EFFICIENCY AND GENETIC RECOMBINATION IN PNEUMOCOCCAL TRANSFORMATION

A tentative molecular model is proposed to explain variation in integration efficiency and the relationship of recombination frequencies to integration and to distance of separation in by DNA from another mutant strain.

Abstract

CONSIDERABLE progress has been made toward an understanding of the molecular basis of transformation in pneumococcus.DNA particles outside the cell compete for entry (HOTCHKISS 1957a).Competent cells indiscriminately take up this DNA at a steady rate to the extent of as much as 5% of their own genetic material (Fox and HOTCHKISS 1957).Immediately following uptake, donor DNA loses its ability to transform other cells (Fox 1960).This appears to result from the conversion of native donor DNA to single strands upon uptake (LACKS 1962).Within one quarter of a generation donor material recovers its activity, apparently as a consequence of insertion of single-stranded segments into native DNA of the host (LACKS 1969;Fox and ALLEN 1964).Simultaneous with this recovery, genetically recombinant DNA appears (Fox 1960).The mechanism of donor marker integration and recombination is less well understood.A prior analysis of several of the mutations reported here indicated that integration efficiencies for different markers may vary considerably and that recombination frequencies depend on integration efficiencies as well as on the physical distance between markers (LACKS and HOTCHKISS 1960a).EPHRUSSI-TAYLOR, SICARD and KAMEN (1965) have recently examined the problem of integration efficiency with a series of mutations in the amiA locus of pneumococcus.The present report consists of an analysis of mutations in the amylomaltase locus of pneumococcus with respect to (a) specific mutagenic origin and reversal, (b) integration frequencies, (c) recombination frequencies between pairs of markers, and (d) relationship of recombination to genetic location.Integration frequencies are interpreted as reflecting specific base differences in donor and recipient DNA.A tentative molecular model is proposed to explain variation in integration efficiency and the relationship of recombination frequencies to integration efficiencies and to distance of separation in DNA.MATERIALS A N D METHODS Bacterial strains: The wild-type strain of pneumococcus, R6, as well as derivatives of this strain bearing markers sulf-d and str-r were obtained from DR. R. D. HOTCHKISS.sulf-d corresponds to the d marker for sulfonamide resistance of HOTCHKISS and EVANS (1958).It was introduced into various mutant strains for purposes of reference.The streptomycin-resistance marker, str-r, has been designated sir-r41 by ROTHEIM and RAVIN (1964).Media The medium used for growth, transformation, and selection was based on that of Research carried out at Brookhaven National Laboratory under the auspices of the U S .Atomic Energy Commission.