Catalyzes the reversible phosphorylation of UMP to UDP (PubMed:30409856). Required for specific post-transcriptional processes of many plastid transcripts (e.g. PSI (PsaA, PsaF), PSII (D1, CP43, CP47), Cytochrome b(6)f (Cytb(6)), ATP synthase (AtpC), LHCs (LHCa3, LHCb2), and NDH (NdhH)), thus being essential for retaining photosynthetic activity in chloroplasts (PubMed:19037728, PubMed:30409856). Associates with group II introns of the plastid transcripts trnG-UCC, trnV-UAC, petB, petD and ndhA to stabilize corresponding precursor RNAs (PubMed:30409856). ATP + UMP = ADP + UDP Pyrimidine metabolism; CTP biosynthesis via de novo pathway; UDP from UMP (UMPK route): step 1/1. Homomultimer (PubMed:30409856). Homohexamer (By similarity). Forms RNA-containing megadalton-sized complexes (PubMed:30409856). Expressed exclusively in leaves, but not in roots. Induced by light, detected at low levels in etiolated seedlings. Reduced capacity to grow photoautotrophically associated with low levels of many plastid transcripts (e.g. PSI (PsaA, PsaF), PSII (D1, CP43, CP47), Cytochrome b(6)f (Cytb(6)), ATP synthase (AtpC), LHCs (LHCa3, LHCb2), and NDH (NdhH)) but abnormal accumulation of others, altered plastid translation as well as a strongly affected plastid ultrastructure (e.g. almost absent stromal lamellae and swollen grana stacks) with a large number of plastoglobuli and reduced photosynthetic performance due to strong defects in the major complexes of the thylakoid membrane. Belongs to the UMP kinase family.