A central challenge of synthetic biology is to enable the growth of living systems using parts that are not derived from nature, but designed and synthesized in the laboratory. As an initial step toward achieving this goal, we probed the ability of a collection of >10(6) de novo designed proteins to provide biological functions necessary to sustain cell growth. Our collection of proteins was drawn from a combinatorial library of 102-residue sequences, designed by binary patterning of polar and nonpolar residues to fold into stable 4-helix bundles. We probed the capacity of proteins from this library to function in vivo by testing their abilities to rescue 27 different knockout strains of Escherichia coli, each deleted for a conditionally essential gene. Four different strains--ΔserB, ΔgltA, ΔilvA, and Δfes--were rescued by specific sequences from our library. Further experiments demonstrated that a strain simultaneously deleted for all four genes was rescued by co-expression of four novel sequences. Thus, cells deleted for ∼0.1% of the E. coli genome (and ∼1% of the genes required for growth under nutrient-poor conditions) can be sustained by sequences designed de novo.