A unique arrangement of promoter elements was found upstream of the bacteriophage P1 particle maturation gene (mat). A P1-specific late-promoter sequence with conserved elements located at positions 22 and 10 was expected from the function of the gene in phage morphogenesis. In addition to a late-promoter sequence, a 35 element and an operator sequence for the major repressor protein, C1, were found. The 35 and 10 elements constituted an active Escherichia coli ⁷⁰ consensus promoter, which was converted into a P1-regulated early promoter by the superimposition of a C1 operator. This combination of early- and late-promoter elements regulates and fine-tunes the expression of the particle maturation gene. During lysogenic growth the gene is turned off by P1 immunity functions. Upon induction of lytic growth, the expression of mat starts simultaneously with the expression of other C1-regulated P1 early functions. However, while most of the latter functions are downregulated during late stages of lytic growth the expression of mat continues throughout the entire lytic growth cycle of bacteriophage P1. Thus, the maturation function has a head start on the structural components of the phage particle.