Tissue-Specific and Light-Dependent Expression within a Family of Nuclear-Encoded Sigma-like Factors fromZea mays1

Abstract

The principal transcription machinery functioning in chloroplasts of higher plants is encoded in two subcellular compartments. Subunits of the RNA polymerase catalytic core are plastid encoded, while sigma factors required for promoter recognition are encoded in the nucleus. We have isolated nuclear-encoded cDNAs,sig1, sig2,andsig3,specifying three sigma factors from maize (Zea mays). The three deduced polypeptides have extensive sequence identity with the principal sigma factors of eubacteria. Two of the maize cDNAs,sig1andsig3,encode NH2-terminal transit peptides which direct the uptake of a heterologous protein into chloroplastsin vitro.Transcripts for thesig3gene were more abundant in green leaves than in roots and in light-treated seedlings than in dark-grown seedlings. In contrast,sig1transcripts were readily detectable in all tissues examined. Thus, at least two promoter-selectivity factors function with the maize chloroplast RNA polymerase, one of which is constitutively expressed and the other is light activated.