The functional significance of noncoding transcripts is currently a major question in biology. We have been studying the function of a set of antisense transcripts called COOLAIR that encompass the whole transcription unit of the Arabidopsis floral repressor FLOWERING LOCUS C (FLC). Alternative polyadenylation of COOL-AIR transcripts correlates with different FLC sense expression states. Suppressor mutagenesis aimed at understanding the importance of this sense-antisense transcriptional circuitry has identified a role for Arabidopsis cyclin-dependent kinase C (CDKC;2) in FLC repression. CDKC;2 functions in an Arabidopsis positive transcription elongation factor b (P-TEFb) complex and influences global RNA polymerase II (Pol II) Ser 2 phosphorylation levels. CDKC;2 activity directly promotes COOLAIR transcription but does not affect an FLC transgene missing the COOLAIR promoter. In the endogenous gene context, however, the reduction of COOLAIR transcription by cdkc;2 disrupts a COOLAIR-mediated repression mechanism that increases FLC expression. This disruption then feeds back to indirectly increase COOLAIR expression. This tight interconnection between sense and antisense transcription, together with differential promoter sensitivity to P-TEFb, is central to quantitative regulation of this important floral repressor gene.lncRNA | autonomous pathway | transcriptional regulation | chromatin silencing W e are investigating the role of specific antisense transcripts in gene regulation through our analysis of the regulation of expression of Arabidopsis thaliana FLOWERING LOCUS C (FLC), an important regulator of flowering time (1, 2). Multiple genetic pathways have been defined that regulate FLC expression; some function in parallel whereas others function antagonistically. FRIGIDA (FRI) up-regulates FLC expression, causing plants to overwinter vegetatively (3). FRI function is antagonized by vernalization, a process through which prolonged cold epigenetically silences FLC (4-7). Acting in parallel with vernalization to repress FLC expression is the autonomous pathway. The autonomous pathway was initially characterized through mutations specifically affecting flowering time but has subsequently been shown to regulate many other targets in the A. thaliana genome (8-10). The autonomous pathway is composed of a number of factors: RNA-binding proteins FCA (11), FPA (12), and FLK (13, 14), RNA 3′ processing factors FY (15-17), CstF64, and CstF77 (18), a histone 3 lysine 4 (H3K4) demethylase FLD (19,20), a homolog of MSI1 (multicopy suppressor of ira1) FVE (21), and a homeodomain protein LUMINIDEPENDENS (LD) (22). These factors link alternative processing of a set of antisense transcripts produced at the FLC locus, collectively termed COOLAIR, with chromatin modifications in the FLC gene body (8). FCA, FY, and FPA promote proximal polyadenylation of COOLAIR transcripts (18), FLD-dependent H3K4me2 demethylation across the body of the gene, and low FLC transcription (23). Their loss results in distal polyadenylation in COOLAI...