Regulation of competence for genetic transformation in Streptococcus pneumoniae: expression of dpnA, a late competence gene encoding a DNA methyltransferase of the DpnII restriction system

The chromosomal DpnII gene cassette of Streptococcus pneumoniae encodes two methyltransferases and an endonuclease. One methyltransferase acts on doub le-stranded and the other on single-stranded DNA. Two mRNAs are transcribed from the cassette. One, a SigA promoter transcript, includes all three gen es; the other includes a truncated form of the second methyltransferase gen e (dpnA) and the endonuclease gene. The truncated dpnA, which is translated from the second start codon in the full gene, was shown to produce active enzyme. A promoter reporter plasmid for S. pneumoniae was devised to charac terize the promoter for the second mRNA. This transcript was found to depen d on a promoter that responded to the induction of competence for genetic t ransformation. The promoter contains the combox sequence recognized by a Si gH-containing RNA polymerase. As part of the competence regulon, the dpnA g ene makes a product able to methylate incoming plasmid strands to protect t hem from the endonuclease and allow plasmid establishment. Its function dif fers from most genes in the regulon, which are involved in DNA uptake. Comp arison of R6 and Rx strains of S. pneumoniae showed the temperature depende nce of transformation in R6 to result from temperature sensitivity of the u ptake apparatus and not the development of competence.