Fig 10. Allele A1 of hsdS A dictates the opaque colony phenotype of S. pneumoniae.
Chromosomal co-expression of hsdS A alleles A1-A3 in the bgaA locus of the ST556 derivatives carrying the locked hsdS A allele A2 (A), A3 (B) or A1 (C). A modified Janus cassette (JC1) was used to replace partially the coding sequence of bgaA in each parent strain. JC1 in the resulting strains were subsequently replaced by the fusion PCR product of the A1, A2, or A3 allele of hsdS A by counter selection, which consisted of the hsdRMS promoter and full coding sequence of each allele. Colonies of each strain were prepared, photographed, and marked as in Fig 6. The genotype and name of each strain are marked at the top and bottom of each photograph. All of the ST556 derivatives carrying allele A1 produced uniformly opaque colonies. In (D), the relationship of the hsdS A allelic variations by DNA inversions and the resulting epigenetic and phenotypic switch is diagrammatically illustrated; the methylated and unmethylated adenine nucleotides in the DNA motif by the HsdA1-associated methyltransferase is highlighted with red and blue characters, respectively. R = A or G, Y = T or C.