Functional analysis of two maize cDNAs encoding T7-like RNA polymerases

We have characterized two maize cDNAs, rpoTm and rpoTp, that encode putativ e T7-like RNA polymerases. In vivo cellular localization experiments using transient expression of the green fluorescent protein suggest that their en coded proteins are targeted exclusively to mitochondria and plastids, respe ctively. An antibody raised against the C terminus of the rpoTp gene produc t identified mitochondrial polypeptides of similar to 100 kD. Their presenc e was correlated with RNA polymerase activity, and the antibody inhibited m itochondrial in vitro transcription activity. Together, these results stron gly suggest that the product of rpoTm is involved in maize mitochondrial tr anscription. By contrast, immunoblot analysis and an antibody-linked polyme rase assay indicated that rpoTp specifies a plastid RNA polymerase componen t. A quantitative reverse transcription-polymerase chain reaction assay was used to study the transcription of rpoTp and rpoTm in different tissues an d under different environmental conditions. Although both genes were consti tutively expressed, rpoTm transcripts were generally more prevalent in nonp hotosynthetic tissues, whereas an increase in rpoTp transcripts paralleled chloroplast development. We suggest that these two genes encode constitutiv e components of the organelle transcription machinery but that their expres sion is nonetheless subject to modulation during plant development.