Abstract
Plastid genes are transcribed by two types of RNA polymerase, an eubacterial-type PEP is responsible for transcription of photosynthesis-related genes, whereas phage-type NEPs are involved in transcription of housekeeping genes. Promoter specificity of PEP is determined by the exchangeable sigma subunit. There are six sigma factor genes homologous to bacterial sigma-70 type sigma factors in Arabidopsis. We have previously reported that AtSIG6 is essential for transcription of many PEP-dependent genes in young seedlings and its mutant (sig6-1) exhibited the pale green phenotype. We found that overexpression of AtSIG1, AtSIG3, and AtSIG4 partially complemented the reduced expression of many PEP-dependent genes in the sig6-1 mutants, whereas AtSIG2 and AtSIG5 preferentially enhanced the expression of some tRNA genes, and a few photosynthesis genes (psbA, psbD and psaA), respectively. Thus, it is likely that some plastid sigma factors have significant overlap in promoter recognition, but other factors recognize a set of special promoters.
