DNA polymerase activity of tomato fruit chromoplasts

Abstract

DNA polymerase activity was measured in chromoplasts of ripening tomato fruits. Plastids isolated from young leaves or mature red fruits showed similar DNA polymerase activities. The same enzyme species was present in either chloroplasts or chromoplasts as judged by PH and temperature profiles, sensitivities towards different inhibitors and relative molecular mass (M 1 88 kDa). The activities analyzed showed the typical behaviour of plastid‐type polymerases. The results presented here suggest that chromoplasts maintain their DNA synthesis potential in fruit tissue at chloroplast levels. Consequently, the sharp decrease of the plastid chromosome transcription observed at the onset of fruit ripening could not be due to limitations in the availability of template molecules. Other mechanisms must be involved in the inhibition of chromoplast RNA synthesis.

Constitutive Transcription and Stable RNA Accumulation in Plastids during the Conversion of Chloroplasts to Chromoplasts in Ripening Tomato Fruits

“…(b) ptDNA could be isolated from both chloro-and chromoplasts (19). The genome copy numbers were fairly the same in plastids from different developmental stages (29). (c) Our purified plastids contained large amounts of plastidspecific RNA (see below), whereas the RNA content of the broken plastid bands was negligible.…”

“…4 and 21). The reasons for this impairment remain obscure, but they do not reflect limiting template availability because ptDNA levels and DNA polymerase activities are kept fairly constant in tomato leaves and fruits (11,29). Inhibition of RNA synthesis might then result from a number of factors, including decrease in the level or the specific activity of RNA polymerase(s) (11, see below) or topological changes that could affect overall template competence (32).…”

“…Moreover, plastids isolated from the high-density band were not leaky to DNase I degradation (see below). (d) Plastid-type DNA polymerase was present in both chloro-and chromoplasts at approximately the same level (29). Following hypotonic lysis of these intact organelles, the activity was entirely recovered in the soluble fraction, indicating that the polymerase was not membrane bound (29).…”

“…(d) Plastid-type DNA polymerase was present in both chloro-and chromoplasts at approximately the same level (29). Following hypotonic lysis of these intact organelles, the activity was entirely recovered in the soluble fraction, indicating that the polymerase was not membrane bound (29). (e) Intact plastid bands contained high levels of RNA polymerase activity, whereas the broken plastid bands did not (E.C.…”

“…therein), nor do they reflect changes in plastid DNA copy number, which is kept fairly constant in the course of chromoplast development (11,29).…”