Saccharomyces cerevisiae is a pleimorphic ascomycete, capable of growing in the yeast form or the pseudohyphal form. The pseudohyphal form is characterized by elongated cells that possess a unipolar budding pattern and grow in chains away from the center of the colony. This change in morphology is induced by nitrogen starvation in the presence of carbon. A collection of 14 elm (elongated morphology) mutants previously have been isolated that grow as pseudohyphae constitutively. Of these mutants, the product of the ELM1 gene was identified and coded for a novel protein kinase. From genetic analyses, ELM1 seemingly functioned downstream of STE7 and STE11, two genes that are essential for pseudohyphal growth. In this study, biochemical characterization of the ELM1 gene product, Elm1p, revealed that Elm1p was a serine/threonine protein kinase. Elm1p autophosphorylated on serine and threonine residues and phosphorylated general phosphoacceptors histone H1, myelin basic protein, and casein. In addition, Elm1p catalytic activity was essential for growth in the yeast form. Elm1p was only detectable in yeast when tagged at the amino terminus. The protein was present in extremely low concentrations and seemed to be tightly regulated, perhaps in a cell cycle dependent manner;Results from morphological analyses showed that, when tagged versions of Elm1p were overexpressed, cell cycle progression was affected. Specifically, cells were blocked in G1 but budding continued. Nuclear division was impaired; nuclei did not divide but instead lodged between the mother and the bud. As a result, cytokinesis was defective. Results from genetic analyses of Elm1p overexpression in the other thirteen elm mutants suggested that ELM1 is epistatic to the other elm mutants;To identify substrates and effectors of Elm1p, the protein-protein interaction trap was employed with Elm1p as the bait. Proteins that interacted with Elm1p in vivo were coded by cDNAs for elongation factor-1[alpha], ribosomal protein (rp) 2L, phosphofructokinase, and two phosphoribosyltransferase homologs. Elm1p also bound to rp 2L in vitro and phosphorylated rp 2L in vitro. Results from this study indicate that Elm1p may regulate pseudohyphal growth through translational modification or nitrogen salvage pathways.