Genes encoding cytochrome_c oxidase subunit 5c from sunflower (Helianthus annuus L.) are regulated by nitrate and oxygen availability

Subunit 5c is the smallest subunit of plant cytochrome c oxidases and has no defined counterpart in animal and fungi. To gain insight about the function and regulation of its expression, we have isolated two cDNAs encoding this subunit from a sunflower (Helianthus annuus L.) root library. The encoded proteins are 91% identical at the amino acid level. Northern blots using specific probes indicated that both genes have similar expression patterns, with higher transcript levels in non-photosynthetic tissues. According to in situ hybridization experiments, expression in roots is localized within developing cortex and vascular tissues, with relatively low levels in the root cap and epidermis. Treatment of plants with solutions containing different nitrate concentrations produced an increase in transcript levels in roots and leaves. Light and/or sugars produced no significant changes in expression. Transcript levels were severely reduced upon incubation under hypoxia, and recovered to initial levels when plants were returned to normoxic conditions. A similar behavior was observed for the cytochrome c gene. We conclude that nitrate and oxygen are important factors regulating the expression of this cytochrome oxidase subunit, which is only partially coordinated with the expression of other components of the cyanide-sensitive respiratory pathway. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.