Fig. 1.

Properties of holins and holin genes. (A) Primary structure of S²¹68 and the N-terminal domains of Lyz, R²¹, and PhoA. The codon numbering for S²¹68 follows that of the full-length gene product, S²¹71 (see C for the dual-start structure at the beginning of S²¹). The SAR sequences of Lyz and R²¹ (7, 8) and TMD1 of S²¹68 (16) are shown in the yellow boxes, with the residues that are polar or neutral in terms of hydrophobicity in blue and the hydrophobic residues in red. TMD2 of S²¹68 is shown in an uncolored box. The red stars separated by three Xs above the TMDs of S²¹ indicate GxxxG-like motifs that may mediate interhelical interactions (19). The vertical arrow above the PhoA sequence indicates the normal signal sequence cleavage site (27). In ^ssPhoAΦS²¹68, the indicated sequence from PhoA is fused to the Met₄ codon of S²¹. The position of the S16C missense change is indicated by a vertical arrow below the TMD1 sequence. The N terminus of S²¹68 was given two additional positive charges by inserting the sequence RYIRS between positions 4 and 5. To generate S²¹68_ΔTMD1, the indicated sequence was deleted from S²¹68. The vertical arrow between residues 66 and 67 indicates the position where the sequence G₂H₆G₂ was inserted in the allele used for purification. (B) Holin topologies. The topologies of the λ antiholin, S107 (a), and holin, S105 (b), are shown (6, 13, 23, 28). (c) and (d) show S²¹71 and S²¹68 with two TMDs, respectively. In (e), the lethal form of S²¹ is shown with its TMD1 in the periplasm. (C) Translational control region of the λ and 21 holin genes (11, 15, 16). Filled and empty stars show starts of the long (antiholin) and short (holin) gene products. Shine-Dalgarno sequences for the first and second translational starts of S^λ are indicated by striped and empty boxes, respectively. The single Shine–Dalgarno sequence of S²¹ is indicated by a striped box. The horizontal inverted pairs of arrows show the RNA stem loops controlling the dual starts. For S21, the vertical arrow shows the Met-4 → Leu mutation, which eliminates the production of the holin from the allele referred to as S²¹71. The Lys residues conferring antiholin character to the longer translational product in both the S and S²¹ genes are shown in red.