The disclosure relates to method of reducing O-glycosylation levels of the insulin or insulin analog precursor molecule produced by Pichia sp. The present disclosure provides genetically engineered knock-out strains of methylotrophic yeast including Pichia and especially Pichia pastoris by disruption of Protein mannosyl transferase (PMT) genes and rendering them capable of producing heterologous proteins with reduced glycosylation. Vectors, which comprise coding sequences for PMT1, PMT2, PMT4, PMT5, and PMT6 genes, for transforming methylotrophic yeasts are contemplated by the present disclosure. PMT inactivated strains of this disclosure have been deposited at MTCC, Chandigarh. The strains are PMT1/GS115 (MTCC 5515), PMT4/GS115 (MTCC 5516), PMT5/GS115 (MTCC 5517) and PMT6/GS115 (MTCC 5518).