Recent studies have shown that Sup35p prion fibrils probably have a parallel in-register β-structure. However, the part(s) of the N-domain critical for fibril formation and maintenance of the [PSI(+)] phenotype remains unclear. Here we designed a set of five SUP35 mutant alleles (sup35(KK)) with lysine substitutions in each of five N-domain repeats, and investigated their effect on infectivity and ability of corresponding proteins to aggregate and coaggregate with wild type Sup35p in the [PSI(+)] strain. Alleles sup35-M1 (Y46K/Q47K) and sup35-M2 (Q61K/Q62K) led to prion loss, whereas sup35-M3 (Q70K/Q71K), sup35-M4 (Q80K/Q81K), and sup35-M5 (Q89K/Q90K) were able to maintain the [PSI(+)] prion. This suggests that the critical part of the parallel in-register β-structure for the studied [PSI(+)] prion variant lies in the first 63-69 residues. Our study also reveals an unexpected interplay between the wild type Sup35p and proteins expressed from the sup35(KK) alleles during prionization. Both Sup35-M1p and Sup35-