FIG. 1.

Schematic diagram of the mrp locus showing the sizes of the predicted open reading frames, the site of the nonpolar mutation made in mutant strain VK1, and the sites of the disruptions made in mutant strains VKN1, VK6, and VK15. Seven open reading frames (mrpABCDEFG) are indicated, using homologues from other bacteria, especially the R. meliloti pha region, as part of the frame of reference. The open reading frames are shown as open boxes, with the direction of transcription indicated by the horizontal arrows within the boxes. An arrow emerging upward from the fragment upstream of mrpA and pointing in the direction of transcription indicates the putative promoter (P). The SphI fragment deleted in VK1 to create a nonpolar in-frame deletion is indicated by the thin bent line flanked by dotted vertical lines. The sites disrupted by a spectinomycin resistance cassette in strains VK6 and VK15 are shown by the open arrows pointing upward. The replacement of the entire mrp locus with a spectinomycin resistance cassette is indicated for VKN1. The primers used to construct mutant strains and integration vectors in this work are shown by horizontal arrows (see Materials and Methods).