Main content area

Characterization of two chloroplast RNA polymerase sigma factors from Zea mays: photoregulation and differential expression

Tan, S., Troxler, R.F.
Proceedings of the National Academy of Sciences of the United States of America 1999 v.96 no.9 pp. 5316-5321
Zea mays, chloroplasts, DNA-directed RNA polymerase, binding proteins, complementary DNA, nucleotide sequences, amino acid sequences, phylogeny, structural genes, multigene family, gene expression, messenger RNA, leaves, light, etiolation, species differences
Two distinct cDNAs encoding putative sigma factors of plastid RNA polymerase were isolated from Zea mays, a C4 plant. The deduced amino acid sequences of both cDNAs possess all four highly conserved domains proposed for recognition of -10 and -35 promoter elements, core complex binding, DNA binding, and melting. These two cDNAs are designated sig1 and sig2. Phylogenetic analysis of available plastid sigma factors indicated that they were probably the descendants of cyanobacterial principal sigma factors. Southern blots probed with sig1 and sig2 revealed that both genes exist in the maize nuclear genome as single-copy genes, but low stringency hybridization suggested the presence of a multi-gene family of maize plastid sigma factors. Transcription of sig1 and sig2 is light inducible and tissue specific. Transcripts of sig1 and sig2 were abundant in greening leaf tissues: sig2 (but not sig1) was barely detectable in etiolated leaves and neither was detectable in roots. Immunological studies using a peptide antibody against an epitope in subdomain 2.4 of Sig1 revealed 50-kDa and 60-kDa immunoreactive proteins in maize chloroplasts. Reduced levels of the 60-kDa immunoreactive protein were detected in etioplasts, and no immunoreactive proteins were observed in roots. Collectively, the data suggest that the nuclear genes, sig1 and sig2, may play a role in differential expression of plastid genes during chloroplast biogenesis.