IscA has been proposed to be a scaffold protein of the iron-sulfur cluster biosynthetic machinery. We have identified the IscA homolog to be localized to plastids, termed AtIscA-I, in Arabidopsis thaliana. The AtIscA-I protein was apparently constitutively expressed in all tissues analyzed in Arabidopsis. The AtTscA-I protein exists in the stroma as a soluble protein which tends to form a homo-dimer and can host a [2Fe-2S]-like cluster. Complete loss of the protein from plastids did not cause any significant defect either in normal plant growth or in biogenesis of major iron-sulfur proteins, indicating this protein is not essential or redundant for these functions. In contrast, loss of one of the three plastid-localized CnfU scaffold proteins, AtCnfU-V, caused significant reduction in the level of AtIscA-I. These data suggest that efficient biogenesis of AtIscA-I scaffold requires function of another essential scaffold protein CnfU. (c) 2005 Elsevier Inc. All rights reserved.