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Anaerobic treatment drastically alters the pattern of protein synthesized by maize primary roots. During the first hour of anaerobiosis, aerobic protein synthesis is halted and there is an increase in the synthesis of a class of polypeptides with approximate molecular weights of 33,000 daltons. During the second hour of anaerobic treatment, the synthesis of another small group of polypeptides is initated. This group, the anerboic polypeptides (ANPs), accounts for > 70% of total protein synthesis after 5 hr of anaerobiosis, and is synthesized in basically the same ratio until root death (approximately 70 hr). The alcohol dehydrogenase polypeptides are major ANPs. RNA isolated from roots treated anaerobically for at least 24 hr directs the translation of only the anaerobic polypeptides. However, RNA from roots treated anaerobically for only 5 hr directs translation of both anaerobic and aerobic polypeptides. Thus an early response to anaerobic treatment is the suppression of aerobic message translation. Although the anaerobic polypeptides share a formal similarity to heat-shock proteins in animals, it is probable that the anaerobic genes are an adaptation to flooding.