INTRODUCTION Nutrient depletion is probably the most common stress that cells encounter and must respond to in order to survive. This is true not only for heterotrophic microorganisms in their natural environment, but also for cells within multicellular animals (e.g., Jang and Hill, 1997). Starvation elicits numerous changes in physiology as cells attempt to re-establish homeostasis. If the starvation conditions persist, the cell may adapt by carrying out a differentiation program that generates new cellular functions appropriate to the new conditions. In many soil microorganisms, such as the yeasts and Dictyostelium, the developmental response to starvation is sporulation. These starvation responses share similar regulatory features such as cell cycle arrest and the expression of subsets of stress response proteins needed for survival or differentiation (Thevelein, 1994; Yamamoto, 1996; Maeda, 1997). We have recently reported that YakA regulates the transition from growth to development in Dictyostelium (Souza et al., 1998). YakA is induced by starvation and is part of a pathway that links nutrient sensing to growth control and the initiation of development. YakA belongs to a subfamily of serine/threonine protein kinases including Yak1p from budding yeast and the Yak-related and minibrain kinases that have been characterized in a number of metazoan species, including humans (Garrett and Broach 1989; Tejedor 1995; Smith et al., 1997). Members of this conserved protein family may be components of a conserved regulatory system governing growth/differentiation transitions in eukaryotes. Relative to wild-type cells, yakA - mutants have a faster cell cycle and smaller cell size and are unable to initiate development. The overexpression of yakA results in cell cycle arrest and rapid early development. A function in cell cycle arrest and cell survival has also been described for Yak1p in yeast where it is part of the response to conditions that activate the Msn stress- response transcription factors (Smith et al., 1998). Yak1p is induced by cell cycle arrest and induces thermotolerance (Garrett et al., 1991; Hartley et al., 1994). Thus, in both S. cerevisiae and Dictyostelium, Yak kinases allow cells to respond to environmental stress by initiating a differentiation program. 3263 Development 126, 3263-3274 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV5321 When nutrients are depleted, Dictyostelium cells undergo cell cycle arrest and initiate a developmental program that ensures survival. The YakA protein kinase governs this transition by regulating the cell cycle, repressing growth- phase genes and inducing developmental genes. YakA mutants have a shortened cell cycle and do not initiate development. A suppressor of yakA that reverses most of the developmental defects of yakA - cells, but none of their growth defects was identified. The inactivated gene, pufA, encodes a member of the Puf protein family of translational regulators. Upon starvation, pufA - cells develop precociously and overexpress developmentally important proteins, including the catalytic subunit of cAMP- dependent protein kinase, PKA-C. Gel mobility-shift assays using a 200-base segment of PKA-C’s mRNA as a probe reveals a complex with wild-type cell extracts, but not with pufA - cell extracts, suggesting the presence of a potential PufA recognition element in the PKA-C mRNA. PKA-C protein levels are low at the times of development when this complex is detectable, whereas when the complex is undetectable PKA-C levels are high. There is also an inverse relationship between PufA and PKA-C protein levels at all times of development in every mutant tested. Furthermore, expression of the putative PufA recognition elements in wild-type cells causes precocious aggregation and PKA-C overexpression, phenocopying a pufA mutation. Finally, YakA function is required for the decline of PufA protein and mRNA levels in the first 4 hours of development. We propose that PufA is a translational regulator that directly controls PKA-C synthesis and that YakA regulates the initiation of development by inhibiting the expression of PufA. Our work also suggests that Puf protein translational regulation evolved prior to the radiation of metazoan species. Key words: Differentiation, Starvation, Cell cycle, minibrain, pumilio, Dictyostelium SUMMARY Starvation promotes Dictyostelium development by relieving PufA inhibition of PKA translation through the YakA kinase pathway Glaucia Mendes Souza 1 , Aline Maria da Silva 1 and Adam Kuspa 2, * 1 Dept. Bioquímica, Instituto de Química, Universidade de São Paulo, 05508-900, Brazil 2 Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA *Author for correspondence (e-mail: akuspa@bcm.tmc.edu) Accepted 29 April; published on WWW 21 June 1999